Abstract: HNPC-A2005, 1-(3-fluoro-4-chlorophenyl) -2-methylthio acetyl ketone oxime-o- (2-methylbiphenyl-3-methyl) ether, is a novel oxime insecticide discovered and developed by Hunan Branch of the National Pesticide R & D South Center. Toxicity, mode of action and field efficacy of HNPC-A2005 was evaluated against the lepidopterous insects. Bioassay results at the laboratory showed that HNPC-A2005 had good stomach, contact, comprehensive action and a weak systemic activity to the lepidopterous insects such as oriental armyworm Mythimna separata and imported cabbageworm Pieris rapae. Quick knockdown action was also observed during the experiment. Greenhouse pot test and field trials demonstrated that HNPC-A2005 exhibited a good control efficacy against imported cabbageworms and tea tussock moth Euproctis pseudoconspersa. HNPC-A2005 had a control efficacy of 93% and 82% against imported cabbageworms and tea tussock moths, respectively, three day after application at the dosage of（a.i.）75 g /hm2, and was safe to beneficial insects and crops.