High temperature effects attributable to climate change can affect rice quality. The chalky area of rice grains is often used to evaluate of rice grain starch quality, but the overall effect of high temperatures on grain chalkiness and overall nutrient quality has not been fully clarified. Thus, in this study, we assessed high temperature effects on grain weight, chalkiness, and nutrient contents. Rice grains were classified into four groups on the basis of the chalky area in scanned grain images: P (0%), S (0–15%), M (15–40%), and L (≥ 40%). Then, the amylose, protein and mineral nutrient concentrations were assessed in each chalkiness classification. High temperatures during grain filling markedly decreased the grain weight and the amylose content of milled rice but increased the chalky area of the grains as well as protein content and the concentrations of most minerals. There were significant negative correlations between mineral contents and both grain weights and amylose contents of milled rice. These results indicate that increases in grain chalky areas due to high temperatures during grain filling also increase grain mineral contents.