Sterile Insect Technique (SIT)/Inherited Sterilely (IS) is considered a promising tactic against Lepidopteran pests using sub-sterilizing doses of ionizing radiation rather than sterile doses. Optimization of irradiation doses is important for developing the SIT/IS operational for an efficacious programme. The present study aimed to evaluate the effective dose which induce sterility for improving SIT/IS against Spodoptera littoralis with particular emphasis on mating ability and competitiveness. Eight gamma radiation doses (0, 100, 125, 150, 175, 200, 225 and 250 Gy) were used to irradiate full-grown male and female pupae of S. littoralis. Results indicated that the female moths were more radiosensitive than male moths, the dose of 175 and 250 Gy exhibited (0.0 %) egg hatch of irradiated female and male moth, respectively. Moreover, the sperm transfer by irradiated male was drastically reduced by increasing the dose level, however, the percentages of spermatophore were increased among the first generation (F₁) of male as compared to their Parent (P₁) at the same dose level. Full competitiveness was attained in the ratio of 3 irradiated: 1un-irradiated male moths at the doses 225 and 250 Gy. Field cage tests revealed that high levels of mating competitiveness were attained between irradiated and non-irradiated moths at all tested doses. Moreover, the copula duration showed insignificant differences among irradiation doses, however the duration was shorter in irradiated category that un-irradiated ones. The dose of 225 Gy caused a full sterility of P₁ female and F₁ male with high competiveness and can be used effectively in SIT/IS for S. littoralis management.