A process for producing a highly purified glucosyl stevia composition, comprising the steps of: adding starch into water to form a starch suspension; adding a mixture of α -amylase and CGTase into the starch suspension and incubating for about 0.5 to 2 hours at about 75-80°C, resulting in a liquefied starch suspension; inactivating the α -amylase by low pH heat treatment; cooling the liquefied starch suspension and adjusting the pH to about 5.5 to 7.0; adding steviol glycosides into the liquefied starch suspension, resulting in a reaction mixture; adding a second batch of CGTase into the reaction mixture and incubating for about 12 to 48 hours at about 55-75°C; adding a second batch of α -amylase into the reaction mixture and incubating for about 12-16 hours at about 55-95°C; inactivating the second batch α -amylase by low pH heat treatment; adjusting the pH to about 5.5 to 7.0; decolorizing the reaction mixture; removing impurities by contacting the decolorized reaction mixture with macroporous adsorbent resin and subsequently eluting adsorbed diterpene glycosides with aqueous ethanol to result in a glycoside-containing aqueous ethanol eluate; desalting the glycoside-containing aqueous ethanol eluate with ion-exchange resins; removing ethanol from the aqueous ethanol eluate, resulting in an aqueous eluate; and concentrating and drying the aqueous eluate to obtain the highly purified glucosyl stevia composition, wherein the highly purified glucosyl stevia composition comprises α -1,4-glucosyl derivatives, and unmodified steviol glycosides, wherein the mixture of α -amylase and CGTase contains about 0.05-0.1 KNU of α -amylase per one unit of CGTase, or wherein the second batch of CGTase has about 0.5-1.2 units of CGTase per gram of solids. adding starch into water to form a starch suspension; adding a mixture of α -amylase and CGTase into the starch suspension and incubating for about 0.5 to 2 hours at about 75-80°C, resulting in a liquefied starch suspension; inactivating the α -amylase by low pH heat treatment; cooling the liquefied starch suspension and adjusting the pH to about 5.5 to 7.0; adding steviol glycosides into the liquefied starch suspension, resulting in a reaction mixture; adding a second batch of CGTase into the reaction mixture and incubating for about 12 to 48 hours at about 55-75°C; adding a second batch of α -amylase into the reaction mixture and incubating for about 12-16 hours at about 55-95°C; inactivating the second batch α -amylase by low pH heat treatment; adjusting the pH to about 5.5 to 7.0; decolorizing the reaction mixture; removing impurities by contacting the decolorized reaction mixture with macroporous adsorbent resin and subsequently eluting adsorbed diterpene glycosides with aqueous ethanol to result in a glycoside-containing aqueous ethanol eluate; desalting the glycoside-containing aqueous ethanol eluate with ion-exchange resins; removing ethanol from the aqueous ethanol eluate, resulting in an aqueous eluate; and concentrating and drying the aqueous eluate to obtain the highly purified glucosyl stevia composition, wherein the highly purified glucosyl stevia composition comprises α -1,4-glucosyl derivatives, and unmodified steviol glycosides, wherein the mixture of α -amylase and CGTase contains about 0.05-0.1 KNU of α -amylase per one unit of CGTase, or wherein the second batch of CGTase has about 0.5-1.2 units of CGTase per gram of solids.