Abstract: Various microwave powers were used to irradiate alfalfa seeds with various time to study the effect of microwave irradiation on nitrogenase activity of endogenous azotobacter and germination of seeds. Germination rate, germination speed and nitrogenase activity of pure cultures that derived from seed-carried azotobacter were tested. The results indicate that: 800 W, 20 s and 500 W, 40 s are found with highest germination rate on the 1^st day, which is 122% and 88.9% times higher than the control group(P<0.01), and highest total germination rate is found within the 14-day period, which is 5.51% and 3.35% times higher than the control group, respectively. Under the two treatments, radical length on the 4^th day is 29.8% and 41.9% times longer than the control group, and more sensitive nitrogenase activity is found on condition of various time than various powers. Short time treatments on condition of the two irradiation powers can increase nitrogenase activity conspicuously, and the treatments that treated more than 32 s make nitrogenase activity lower than the control group, conspicuously. Nitrogenase activity is found 104.9% times higher than the control group on condition of 24 s.