An optimized sugar substitute composition with low glycemic index (GI) and low caloric content is disclosed for substituting for sugar and conventional artificial sweeteners. The sugar substitute composition contains siraitia grosvenorii extracts, i.e. mogrosides, including mogroside V, and erythritol that can cover a bitter after taste of mogrosides and work with mogrosides to create synergy for magnifying sweet taste. A content ratio of mogroside V to erythritol in the composition ranged between 50:50 and 0.1:99.9 is able to dilute siraitia grosvenorii extracts and create synergy to obtain the optimized sugar substitute composition with enhanced sweet taste and low GI value. The optimized sugar substitute composition can be used as good sweetener for foods, beverages, seasonings and various food additives while brings health benefits to users to upgrade the dietary quality of people in the modern society.