Both Oxyopes sertatus and Harpactor fuscipes are important predators of common cutworm (Spodoptera litura) on tobacco. The effect of several interference factors on the predations of O. sertatus and H. fuscipes on the larvae of S. litura were examined in laboratory. The results indicated that there were significant mutual interferences on their predations with the existence of other individual in the same/different species. As the number of the predator increased, the predation rates decreased significantly. The interference coefficients (m) within H. fuscipes and within O. sertatus were 0.7278 and 06911, respectively, while the interference coefficient (m) between O. sertatus and H. fuscipes was 0.9464. These results showed that the effect of mutual interference on predation in interspecies was more obvious than that in intraspecies. The number of prey captured and the predation rate of predator dropped with increasing the number of tobacco stalks. This result suggested that spatial heterogeneity was also an important factor affecting the predation of predator on prey.