Abstract: In order to determine the releasing proportion and field control efficacy of Sterile Insect Technique (SIT) against Heliothis assulta, a mating competition experiment of sterile H. assulta was conducted, and the density of male offspring in the field was monitored by using sex pheromone trap and the control efficacy was investigated. The main results were as follows:Firstly, when X-ray irradiation at 144 Gy was applied to treat pupae of H. assulta, there was no significant difference on the eclosion rate between treated and normal male pupae, but the adult longevity decreased significantly. Secondly, the releasing sterile male adults of H. assulta at different rates had no significant influence on the mating rate and fecundity of normal females, but the egg hatching rate decreased with the increase in releasing proportion of irradiated H. assulta according to the mating competition trials. Thirdly, the releasing proportion at 8:1 of H. assulta (traeted:normal) resulted in high average control efficacy (91.11%) in the field cage experiment, which was more efficient than the conventional control method. Fourthly, the releasing proportion at 8:1 could significantly reduce the number of male adult in the field, and the control efficacy was 64.86%. In this study, the releasing proportion at 8:1 was the optimal proportion for induced fertility of H. assulta, which had significant effects on control of adult. The present study strongly supports the conclusion that irradiated sterile males could be released in the environment to reduce H. assulta populations in targeted areas. The X-ray irradiation dose of 144 Gy seems promising as it sterilizes pupae without reducing their emergence and fecundity of normal females. The releasing proportion of 8:1 could be applied in H. assulta prevention and control in the field. SIT, as an effective and environment-friendly control tactic, can be applied as part of area-wide integrated pest-management (AWIPM) program in the control of H. assulta.