diff --git a/spec/internals/tooltip-spec.js b/spec/internals/tooltip-spec.js
index faa28a894..e8bfd56e8 100644
--- a/spec/internals/tooltip-spec.js
+++ b/spec/internals/tooltip-spec.js
@@ -239,6 +239,33 @@ describe("TOOLTIP", function() {
 			expect(top).to.be.equal(tooltipPos.top);
 			expect(left).to.be.equal(tooltipPos.left);
 		});
+
+		it("set option tooltip.position", () => {
+			args.tooltip.position = () => ({top: "10%", left: 20});
+		});
+
+		it("check tooltip's position unit", () => {
+			const pos = args.tooltip.position();
+			util.hoverChart(chart);
+
+			["top", "left"].forEach(v => {
+				expect(chart.$.tooltip.style(v)).to.be.equal(
+					pos[v] + (v === "left" ? "px" : "")
+				);
+			});
+		});
+
+		it("set option tooltip.position={unit: '%'}", () => {
+			args.tooltip.position = {unit: "%"};
+		});
+
+		it("check tooltip's position unit as percentage", () => {
+			util.hoverChart(chart);
+
+			["top", "left"].forEach(v => {
+				expect(/^\d+(\.\d+)?%$/.test(chart.$.tooltip.style(v))).to.be.true;
+			});
+		});
 	});
 
 	describe("tooltip order", () => {
diff --git a/src/config/Options.js b/src/config/Options.js
index 711acb211..601b5b6d1 100644
--- a/src/config/Options.js
+++ b/src/config/Options.js
@@ -3723,8 +3723,10 @@ export default class Options {
 			 * @property {Function} [tooltip.format.value] Set format for the value of each data in tooltip.<br>
 			 *  Specified function receives name, ratio, id and index of the data point to show. ratio will be undefined if the chart is not donut/pie/gauge.
 			 *  If undefined returned, the row of that value will be skipped.
-			 * @property {Function} [tooltip.position] Set custom position for the tooltip.<br>
+			 * @property {Function} [tooltip.position] Set custom position function for the tooltip.<br>
 			 *  This option can be used to modify the tooltip position by returning object that has top and left.
+			 * @property {String} [tooltip.position.unit="px"] Set tooltip's position unit.
+			 *  - **NOTE:** This option can't be used along with `tooltip.position` custom function. If want to specify unit in custom function, return value with desired unit.
 			 * @property {Function|Object} [tooltip.contents] Set custom HTML for the tooltip.<br>
 			 *  Specified function receives data, defaultTitleFormat, defaultValueFormat and color of the data point to show. If tooltip.grouped is true, data includes multiple data points.
 			 * @property {String|HTMLElement} [tooltip.contents.bindto=undefined] Set CSS selector or element reference to bind tooltip.
@@ -3772,7 +3774,14 @@ export default class Options {
 			 *          value: function(value, ratio, id, index) { return ratio; }
 			 *      },
 			 *      position: function(data, width, height, element) {
-			 *          return {top: 0, left: 0}
+			 *      	// return with unit or without. If the value is number, is treated as 'px'.
+			 *      	return {top: "10%", left: 20}  // top:10%; left: 20px;
+  			 *      },
+			 *
+			 *      position: {
+			 *      	// set tooltip's position unit as '%', rather than 'px'.
+			 *      	// ex) If want to keep the position on mobile device rotation, set as '%'.
+			 *      	unit: "%"
   			 *      },
 			 *
   			 *      contents: function(d, defaultTitleFormat, defaultValueFormat, color) {
diff --git a/src/internals/tooltip.js b/src/internals/tooltip.js
index 25d373f80..47642902c 100644
--- a/src/internals/tooltip.js
+++ b/src/internals/tooltip.js
@@ -287,7 +287,6 @@ extend(ChartInternal.prototype, {
 		const bindto = config.tooltip_contents.bindto;
 		const forArc = $$.hasArcType(null, ["radar"]);
 		const dataToShow = selectedData.filter(d => d && isValue($$.getBaseValue(d)));
-		const positionFunction = config.tooltip_position || $$.tooltipPosition;
 
 		if (dataToShow.length === 0 || !config.tooltip_show) {
 			return;
@@ -295,8 +294,7 @@ extend(ChartInternal.prototype, {
 
 		const datum = $$.tooltip.datum();
 		const dataStr = JSON.stringify(selectedData);
-		let width = (datum && datum.width) || 0;
-		let height = (datum && datum.height) || 0;
+		let {width = 0, height = 0} = datum || {};
 
 		if (!datum || datum.current !== dataStr) {
 			const index = selectedData.concat().sort()[0].index;
@@ -325,13 +323,35 @@ extend(ChartInternal.prototype, {
 		}
 
 		if (!bindto) {
+			let fn = config.tooltip_position;
+			let unit;
+
+			if (!isFunction(fn)) {
+				unit = fn && fn.unit;
+				fn = $$.tooltipPosition;
+			}
+
 			// Get tooltip dimensions
-			const position = positionFunction.call(this, dataToShow, width, height, element);
+			const pos = fn.call(this, dataToShow, width, height, element);
 
-			// Set tooltip position
-			$$.tooltip
-				.style("top", `${position.top}px`)
-				.style("left", `${position.left}px`);
+			["top", "left"].forEach(v => {
+				let value = pos[v];
+
+				// when value is number
+				if (/^\d+(\.\d+)?$/.test(value)) {
+					if (unit === "%") {
+						const size = $$[v === "top" ? "currentHeight" : "currentWidth"];
+
+						value = value / size * 100;
+					} else {
+						unit = "px";
+					}
+
+					value += unit;
+				}
+
+				$$.tooltip.style(v, value);
+			});
 		}
 	},
 
diff --git a/types/options.d.ts b/types/options.d.ts
index 78054ef5c..1edb9ab56 100644
--- a/types/options.d.ts
+++ b/types/options.d.ts
@@ -861,15 +861,18 @@ export interface TooltipOptions {
 	order?: string | any[] | ((data1: any, data2: any) => number) | null;
 
 	/**
-	 * Set custom position for the tooltip.
+	 * Set custom position function for the tooltip.
 	 * This option can be used to modify the tooltip position by returning object that has top and left.
+	 *
+	 * Or set tooltip's position unit.
+	 * This option can't be used along with `tooltip.position` custom function. If want to specify unit in custom function, return value with desired unit.
 	 */
 	position?(
 		data: any,
 		width: number,
 		height: number,
 		element: any
-	): { top: number; left: number };
+	): { top: number; left: number } | { unit: string; };
 
 	/**
 	 * Set custom HTML for the tooltip.