The objective of this study was to clarify the effect of temperature during initial ripening stage and assimilation in chalky rice grain. Japonica rice cultivars: Fusaotome, Fusakogane, Akitakomachi, Haenuki, Sasanishiki, and Koshihikari, showed different percentages in the occurrence of chalky rice grain under high temperatures during the initial ripening stage. The above Japonica rice cultivars were used to clarify the order of all grains within a panicle, and investigate rice grain weight and rice quality at different spikelet positions. Significant negative correlation between the flowering date and rice grain weight in all cultivars suggests that rice grain weight is affected by the length of the ripening period. There was also a significant positive correlation between the percentage of chalky rice grain and the days from heading to flowering in Haenuki and Sasanishiki cultivars. However, a significant negative correlation was found between the percentage of chalky rice grain and days from heading to flowering in Fusaotome, Fusakogane and Akitakomachi. In addition, the daily average temperature during initial ripening stage in Fusaotome, Fusakogane and Akitakomachi was higher than the other cultivars. These results suggest that the days from heading to flowering within a panicle and high temperature at initial ripening period cause the occurrence of chalky rice grain.