Two new (1 and 2), together with four known (3-6) alkaloids were isolated from the whole plants of Thalictrum microgynum. Compounds 1-6 were tested for their anti-tobacco mosaic virus (anti-TMV) activity. The results indicated that compounds 2-4 showed high anti-TMV activity with inhibition rates of 34.8, 42.1, and 35.2% at a concentration of 20 mu M, respectively. These rates were higher than that of ningnanmycin, a positive control. Compounds 1, 5, and 6 also showed potential anti-TMV activity with inhibition rates of 26.7, 24.8, and 28.2%.