The present invention relates to a plant-based extract and fraction from Nephelium lappaceum rind with good antiglycemic activity namely alphas glucosidase and a method of preparation thereof. Partially purified fractions of the rind having alpha-glucosidase, antioxidant and significant high phenolic content were established using preparative HPLC. These extracts, alone or in combination with other active principles, are applicable to be used as antidiabetic, antiglycemic, slimming or antiobesity agents.