The net photosynthetic rate and chlorophyll fluorescence parameters of flag leaves were measured under high temperature by using the hightemperature tolerant line 996 and hightemperature sensitive line 4628. Rice plants were subjected to high temperature（8:00-17:00,37℃; 17:00-8:00,30℃）treatment for seven days after heading. The net photosynthetic rates of flag leaves of the two rice lines decreased dramatically under the high temperature stress, with a greater decrease in the hightemperature sensitive line 4628 than in the hightemperature tolerant line 996; less accumulation of excess excitation energy in the reaction centers did less harm to the centers and the antenna systems in 996 under high temperature stress. Accordingly, higher qP and qN in 996 helped to dissipate the excess excitation energy and protect the reaction centers from heat injury. However, in 4628, qP declined rapidly, while Ex increased as the high temperature stress persisted. This indicates that the reaction centers and antenna systems in 4628 were damaged severely under high temperature stress, which led to the lower photosynthetic rate.