Bioassay was arranged with uniform experimental design method to study the combined synergism of cypermethrin and quinalphos on the 2nd instar larvae of common cutworm, Spodoptera litura . The relationship between the death probability of target insect and the dosages of the insecticides was described as a quadratic polynomial. The mathematical relationship between co toxicity coefficient (CTC) and the dosages of the insecticides (N 1, N 2 ) was deduced from the calculating formula of CTC, i.e. CTC=100/[(N 1/a) (N 2/b)], where a, b represent the medium lethal concentrations of cypermethrin and quinalphos, respectively. The formula: j=(N 1/a) (N 2/b) , was taken as the goal function of optimization problem. Finally, the maximum CTC and optimal mixture ratio of cypermethrin and quinolphos were obtained by using the program, NLP of SAS.