Temperature rise by global warming causes deterioration in rice yield and grain quality. This study was conducted to investigate the changes in rice panicle-related traits under the high temperature condition. A total of 40 Korean rice cultivars including 27 japonica and 13 Tongil-type (japonica-indica cross) were cultivated in two plots in the field. Eight weeks after transplanting, a greenhouse was set over one plot for the high temperature treatment. Plants were harvested upon maturity and 13 panicle-related traits were investigated. In both ambient and high temperature conditions, Tongil-type showed higher sink size-related characters than japonica, while japonica had higher percent ripened grain and percentage of head rice than Tongil-type. In correlation analysis among the panicle-related traits in both conditions, the number of secondary rachis branches (SRBs) per panicle and the number of spikelets on SRBs per panicle were positively correlated with the numbers of spikelets and rachis-branches per panicle (r > 0.95) while negatively correlated with the percent ripened grain (r < -0.40). Under the high temperature, all panicle-related traits except for the number of primary rachis-branches (PRBs) per panicle were significantly different from those in the ambient condition. On ecotype level, numbers of spikelets and rachis-branches per panicle, the number of SRBs per panicle, and the number of spikelets on SRBs per panicle were significantly increased in Tongil-type under the high temperature condition, while those in japonica did not show any significant change. However, percent ripened grain and the percentage of head rice significantly decreased under the high temperature in both ecotypes. Interestingly, decrease in the percent ripened grain was more severe in the spikelets on SRBs than PRBs in both ecotypes and Tongil-type showed significantly higher decrease in the percent ripened grain than japonica. Therefore, it is concluded that the sink size of Tongil-type cultivars were increased under the high temperature more than japonica cultivars by the significant increase in SRBs, which caused more severe reduction in percent ripened grain.