Abstract: A series of laboratory bioassays was conducted to determine the effects of various operational factors on the deposition and the responses of 2nd instar larvae of tobacco cutworm, Spodoptera litura , to three insecticides, chlorpyrifos, methomyl and beta cypermethrin in the petri dish Potter tower method. Settling time could affect the deposition. Toxicities of chlorpyrifos on the tested insects were not significantly different between 15, 30, 60, 120 s settling times. The spray volume (0 5-4 0 mL) and pressure (25-200 kPa) significantly influenced the deposition and the responses of tobacco cutworm to insecticides. For the three insecticides, LC 50 of a 4 0 mL spray volume was 11, 3 and 5 fold more toxic than that of a 0.5 mL spray volume, respectively. However, the influences of pressure on the deposition and LC 50 were not so serious as volume, and the difference of between pressures was less than 2 fold. The optimal spray conditions in Potter tower method were suggested.