Some extracts of medicinal plants have been proven to be beneficial in treating a number of diseases for centuries such as treating diabetes, antioxidant, anti-inflammatory, antibacterial, etc.This study aimed to evaluate the effect of extracts from several herbal plants, such as Gymnema sylvestre, Stevia rebaudiana, Cinnamomum cassia, Zea may, Ocimum basilicumon on blood glucose level in streptozotocin (STZ) induced diabetic mice and to offer scientific proofs for the identified antihyperglycemic effect by investigating on mechanisms of the most effective extract.The results proved that diabetic mice treated with 70% ethanol extracts of Gymnema sylvestre leaves and Stevia rebaudiana leaves showed significant reduction of the blood glucose levels at a dose of 500 mg/kg body weight when compared to control (P < 0.05). Antihyperglycemicactivity of Gymnema sylvestre (57.68%) and Stevia rebaudiana extracts (54.93%) was significantly higher than those of other extracts. The inhibition of α-amylase and α-glucosidase activity of Gymnema sylvestre and, Stevia rebaudiana extracts were carried out in vitro. The results demonstrated that these Gymnema sylvestre and Stevia rebaudiana extracts were able to strongly inhibit the activity of α-glucosidase and α-amylase, with the IC50 values lower than the recently published values around the world about 2−5 times. Using 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay showed that Gymnema sylvestre and Stevia rebaudiana extracts exhibited relatively low antioxidant activity with the concentration of a sample required for 50%scavenging of the DPPH free radical of 115.88 1.16 µg/mL and 160.27 2.01 µg/mL compared to vitamin C (49.16 1.26 µg/mL).