A series of experiments was conducted to determine the toxicities of thiodicarb on the six lepidopterous pests (Pseudaletia separata, Plutella xylostella, Palpita indica, Spodoptera exigua, Helicoverpa assulta, Spodoptera litura) and to elucidate factors insecticidal effects mechanism of thiodicarb. Thiodicarb was very effective against six lepidopterous young larvae, but less effective to the old larvae and it acted slowly. Thiodicarb inhibited acetylcholinesterase and glutathione S-transferase activities, but not inhibit esterase activity.