Abstract: Euseius utilis is a generalist and distributed widespreadly in northern area of China.The function response and interference of female adults of predatory mites E.utilis on Tetranychus truncatus and 1st instar larvae of Frankliniella occidentalis was determined in the laboratory in order to evaluate its potential for a biological control agent.The results showed that the functional responses for adults of Euseius utilis on T.truncatus and 1st instar larvae of F.occidentalis could be described as Holling Ⅱ type.E.utilis exhibited higher ability to control larvae and nymph of T.truncatus and 1st instar larvae of F.occidentalis whereas the lowest ability to adults of T.truncatus.With a fixed number of prey available,the average predation decreased with predator density increased by mutual competition and interference. E.utilis is promised to be one of the potential biological control agents for T.truncatus and F.occidentalis.