Objective: In vitro analysis of the anti-diabetic effect of aqueous extracts of the medicinal plants Nigella sativa, Eugenia jambolana, Andrographis paniculata, Gymnema sylvestre. Methods: Aqueous extracts of the plants were prepared by maceration. They were then tested for inhibition of α-amylase activity by DNSA colour reagent. They were tested for their ability to hinder diffusion of glucose across a dialysis membrane. Results: The aqueous extract of Nigella sativa showed maximum inhibition of α-amylase activity and a strong hindrance to diffusion of glucose across a dialysis membrane. Andrographis paniculata showed both a strong inhibition of α-amylase and a significant hindrance to the diffusion of glucose across the dialysis membrane. Gymnema sylvestre showed low inhibition of α-amylase activity, but it showed maximum hindrance to the diffusion of glucose across the dialysis membrane. Nigella sativa was found to possess maximum anti-diabetic properties. Conclusions: The findings indicate that all the above plants possess antidiabetic properties too varying degrees. They can be used to develop natural drugs which may be used in lieu of commonly used strong allopathic drugs which possess a number of harmful side effects. © 2013 IJDDR, A. Sathiavelu et al.