The effect of pendimethalin, a herbicide, on microbial activity in soil was studied here. The results showed that the amount of emission CO2 from three treatment of 1, 5 and 10 micrograms.g-1 of pendimethalin had no significant differences from the CK. The study of the influences on the degradation of the cellulose demonstrated that pendimethalin can stimulate the cellulose degradation in the soil. In contrast to the CK, the half-life (T0.5) of 5, 10 micrograms.g-1 treatments were increased to 1.32 times and 2.14 times, respectively. The activities of dehydrogenase were activated in some degrees, and the higher the pendimethalin concentration the greater and longer the influence. The results also showed that the soil urease activity was restrained, and then, was stimulated by pendimethalin. The level and the sustained time of inhibition and stimulation was proportional to the concentration of the treatment.