To evaluate the combined activity of spinosad (spinosyns A and D) and spinetoram (spinosyns J and L) against the adult of two stored grain insects, Sitophilus oryzae and Tribolium castaneum, laboratory bioassays were conducted for each insecticide alone at five concentration levels, and at four levels for the binary mixture. The results indicated that the values of Co-toxicity Factor were affected by the mixture concentration level, the exposure interval and the insect susceptibility. They ranged from +344.4 (a potentiation effect) at the highest mixture concentration level on S. oryzae, after one day exposure time, to -14.29 (an additive effect) at the lowest mixture concentration level, which gave the least adult mortality of T. castaneum after 14 days exposure time. It was concluded that the activity of the spinosad and spinetoram mixture will increase at high concentrations, for short exposure times and against susceptible insects.