[Objectives]  To determine the optimal duration of exposure to UV irradiation necessary to kill Sitotroga cerealella embryos. [Methods]  The lethality to S. cerealella embryos of 5 to 25 min exposure (5 min increments) to ultraviolet light was measured. The effect of exposure durations of 5 to 40 min (5 min increments) and 4 different distances between eggs and UV lamps on hatching rate was also compared. [Results]  The lethality of UV lamps was significantly higher when these were employed for shorter periods of time. Duration of exposure and relative position had significant effects on hatching rate. Overall, the shorter the irradiation time and the greater the distance between eggs and the light source, the lower the embryo mortality rate. The lowest hatching rates, 0.63% and 0.77%, were achieved by irradiating eggs for 25 and 40 min, respectively. Once UV irradiation exceeded 15 min there was no significant change in hatching rate; the highest hatching rate under these conditions was 6.22%. Eggs that had been exposed to UV light were similar in color to control eggs. Like control eggs, they gradually turned reddish-brown but most did not hatch. [Conclusion]  Ultraviolet irradiation does not kill S. cerealella embryos immediately and the lethality of UV lamps decreased significantly after being used for a certain time. The optimal treatment time, using the device developed in this study, is 20-30 min.