Ginkgo biloba leave extract having novel composition useful for prevention or treatment of alzheimer's disease and parkinson's disease, and extracting and purifying method thereof

A ginkgo biloba extract and a pharmaceutical composition comprising the same are provided to show excellent therapeutic effect on brain dysfunction such as Alzheimer's disease and Parkinson's disease and nitric oxide generation inhibition effect. The ginkgo biloba extract for preventing and treating Alzheimer's disease and Parkinson's disease comprises 10-14 wt.% of terpene trilactone, 20-30 wt.% of ginkgoflavone glycoside, and less than 5 ppm of ginkgolic acid. The pharmaceutical composition comprises 20-240 mg of the ginkgo biloba extract per unit formulation, further comprises a pharmaceutically acceptable carrier selected from the group consisting of a diluent, an excipient, a disintegrating agent, a binder, and a lubricating agent and is formulated into a formulation selected from the group consisting of powders, granules, tablets, soft capsules, hard capsules and injections.