The present invention discloses an alpha-glucosidase inhibitor consists of the combination of Lactobacillus plantarum and a biotransformed Momordica charantia juice. Besides, the present invention discloses a method to produce the alphaglucosidase inhibitor comprising steps of preparing a Momordica charantia juice, preparing Lactobacillus plantarum inoculums and fermenting the Momordica charantia juice with the Lactobacillus plantarum inoculums. The alpha-glucosidase inhibitor is able to be used as a medicament to treat diabetes or as a health supplement for preventing diabetes.