Male and female litchi stem-end borers, Conopomorpha sinensis (Lepidoptera: Gracillariidae) were γ-irradiated in a 60Co source during several periods of their pupal development and as newly emerged adults. When mature pupae, the most suitable stage for irradiation, received increasing γ-radiation doses, the emergence rate, flight ability and adult longevity were increasingly diminished. Females that emerged from mature pupae irradiated with 200, 250, and 300 Gy did not oviposit any eggs when mated with either non-irradiated males (UM) or treated males (TM), indicating that 200 Gy was a sufficient dose for inducing complete sterility in females. The hatch rates of eggs oviposited by non-irradiated females (UF) mated with males irradiated with either 200, 250 or 300 Gy were 31, 13.5 and 0.67%, respectively. However, if the father—parental generation (P)—of the male in the cross, UF × F₁ M, had been irradiated with either 200, 250 or 300 Gy, then the percent hatch of the eggs produced was either 9.37 ± 1.68, 0 or 0%, respectively; which indicated that 250 and 300 Gy applied to P generation males resulted in complete sterility in F₁ males. The sexual competitiveness (C) of male litchi stem-end borers that had been irradiated with 250 Gy ranged between 0.53 and 0.62 as measured by ratio tests in laboratory cages with UF:UM:TM ratios of 1:1:1, 1:1:3 and 1:1:5. In a field cage experiment, a mean C value of 0.48 was observed when males were treated with 250 Gy. The C values obtained suggest that males treated with 250 Gy could compete adequately with wild litchi stem-end borers both under laboratory and field cage conditions to warrant pilot scale field tests.