In Japan farmers follow the custom of exposing grain to the sun's heat after passing of the rainy season to destroy insect infestation and to preserve it afterwards. The effect of this practice on the mortality was investigated for the various developmental stages of the rice weevil in kernels of naked barley. The mortality index (D per cent) was calculated as follows : D= C / C-Cx x 100 In this expression C and Cx represented the numbers of adult emerging from the untreated and treated samples respectively. The relationship between calculated percentage mortality index (y) and amount of insolation (x) received during the period of exposing was found to be expressed in the equation, y = axb, where a and b were calculated constants. First to third instar larvae were relatively sensitive to insolation and showed measurable mortality by the shortest exposing of ten minutes, while fourth instar larvae and pupae had no, effect of that exposing. All developmental stages of insect were killed by about 100 cal. amount of insolation. The effect of exposing to sun's heat on germination of naked barley was also investigated. There were no significant differences in gemination among control and experiments including five hours exposing.