Cotton bollworm, Helicoverpa armigera Hub, is one of the serious pests of the several agricultural products. In this study, lethal and sublethal effect of insecticide of dichlorvos was evaluated on 3rd instar larvae of the insect by contact residue method and oral method. All bioassay tests were carried out at 25±2°c, 60±5% RH and a photoperiod of 16:8 (L: D) h. The LC50 and LC30 values were 884.1 and 590.3 mg.ai/ml in contact residue method and 218.4 and 125.3 mg.ai/ml in oral exposure method, respectively. In sublethal effects study, the third larvae were individually exposure time to LC30 of the insecticide in contact residue and oral method were 24 h and 48 h, respectively. and then biological parameters of survivors were evaluated. The result showed that LC30 of the dichlorvos had a significant effect on pupal weight, larval period and pupal period compared with control. The means of larval development period in control, contact residue and oral treatments were 10.9, 17.8 and 13.1 Days, means of pupal weight 0.3, 0.26 and 0.27 g and means of pupal period 14.5, 13.3 and 13.1 days, respectively. Mean number of eggs per female were 176.4, 153.4 and 108.5 eggs and percentage of eggs hatching were 96.1, 56.7 and 56.2% in control, contact residue and oral methods, respectively. Longevity of male and female adult was 6.8, and 6.9 days that were reduced compared to control with 8.8 days. Successful completion of larval stage was reduced by 18.6% in contact residue method compared to control.