Grain filling in rice, a staple cereal crop worldwide, is a critical determinant of grain yield and quality. However, there is little available information on the relationship between grain filling and grain photosynthetic capacity in rice. This study evaluated the genetic diversity among six rice cultivars for their grain filling rate (GR0) and the relationships with the grain chlorophyll contents and grain net photosynthetic rate (PN). Significant variations in GR0, PN, and the chlorophyll contents (a, b, and total) in the grains of the cultivars were observed. Approximately 90 % of the variation in GR0 was explained by the grain PN. General linear model regression revealed significant positive correlations between PN/GR0 and the chlorophyll contents (a, b, and total) in the grains. There was also a significant positive correlation between PN and GR0. These positive correlations suggest a direct positive relationship between the grain filling rate and grain chlorophyll contents, which is indicative of the high photosynthetic capacity of the grains during the early grain filling period. These results suggest that the grain chlorophyll contents could be used as a molecular marker in marker-assisted breeding programs for rice cultivars with high grain net photosynthetic capacity during the early period of grain filling rate to improve grain yield.