Abstract: Taking two rice lines including heattolerant line 996 and heatsensitive line 4628 as materials，the experiment was conducted in growth chamber to investigate the effects of high temperature（9：00–17：00，37°C;17：00–9：00，30°C）and optimal temperature（9：00–17：00，30°C;17：00–9：00，25°C）on photosynthesis characteristics，membrane permeability，antioxidant enzymes activities in flag leaves during grainfilling period and grain quality.The results showed that chlorophyll content，chlorophyll a/b，net photosynthetic rate of flag leaves in rice decreased，the activities of superoxide（SOD）and peroxidase（POD）in flag leaves increased at early stage with high temperature stress，then decreased during grainfilling period.The decreasing range of chlorophyll content，net photosynthetic rate，SOD and POD activity in flag leaves of heat sensitive line 4628 was greater than those of heat tolerant line 996.MDA content and relative conductivity in flag leaves increased with high temperature stress，the increasing range of MDA content and relative conductivity in flag leaves of heat sensitive line 4628 was greater than those of heat tolerant line 996.With high temperature stress，milled rice rate，head rice rate，amylose content of both lines decreased at grain filling stages，while chalkness rate，chalkness，protein content increased.The main reason of grain quality degeneration under high temperature stress included antioxidant enzymes activities decreasing，membrane permeability increasing，photosynthesis capacity and transport and unloading capacity of photosynthetic products in function leaves decreasing.

Key words: Grain filling stage, High temperature stress, Flag leaves, Physiological characteritcs, Grain quality