Lagerstroemia indica L. has a high medicinal value, its bark, leaves and flowers can be used as medicine. Its flowers are reddish, purple, or white, thereinto, the white one is called L. indica Linn. f. alba (Nichols.) Rehd., which is a forma of L. indica. In this paper, the hypoglycemic effects of different extracts from flowers of L. indica (LIF) and L. indica L. f. alba (Nichols.) Rehd. (LIAF) were investigated by α-glucosidase inhibitory method in vitro and alloxan-induced diabetic mice model in vivo, respectively. The results in vitro showed that ethyl acetate extracts (EA) of LIF and LIAF (IC50 = 4.45 and 4.09 µg/mL, resepectively) had the highest inhibitory activity of α-glucosidase, and followed by n-butyl alcohol extracts (BU) (IC50 = 17.01 and 14.58 µg/mL, respectively), and the last was petroleum ether extracts (PE) (IC50 = 103.29 and 112.47 µg/mL, respectively), the activities of all extracts were higher than that of the positive control (acarbose, IC50 = 1 278.83 µg/mL). The results in vivo showed that compared with the model control group, each dose group of LIF could significantly (P < 0.05, P < 0.01 or P < 0.001) decrease fasting blood glucose, total cholesterol (TC) level and malondialdehyde (MDA) content in diabetic mice and increase superoxide dismutase (SOD) level (P < 0.05) in serum. Partial dose groups of LIF could significantly (P < 0.05) decrease postprandial blood glucose, triglyceride (TG) and increase liver glycogen content. Each dose group of LIAF could significantly (P < 0.05, P < 0.01 or P < 0.001) decrease fasting blood glucose, TC level and increase SOD level in serum. Partial dose groups of LIAF could significantly (P < 0.05) decrease TG, MDA level and increase liver glycogen content. These results indicate that LIF and LIAF had effective prevention and treatment effects on the development for diabetes.