Objectives : The objective of the study was to observe the effects of different treatment lengths of Rhizoma Alismatis on diabetes mellitus. Methods : Rats were divided into three groups: normal(non-treated group), control (group administered saline for 4, 11, and 18 days), and RA (group administered 2.45mg/200g Rhizoma Alismatis for 4, 11, and 18 days). The experimental results were derived from the measurement of the levels of glucose, ALP, AST, ALT, creatinine and BUN in the serum from the rats on days 4, 11, and 18. Results : The glucose level in the serum significantly decreased on the 18th day. The ALP level in the serum did not show statistical significance. The GOT and GPT levels in the serum slightly decreased on the 4th, 11th, and 18th days, but did not show statistical significance. The creatinine level in the serum was unchanged for all of them. The BUN level in the serum did not show statistical significance. Conclusions : According to the above results, it is concluded that Rhizoma Alismatis has a therapeutic effect on diabetes mellitus with a longer period ofintake. As to influence upon the liver and kidney, there was no damage orside effects.