Abstract: The changes of chlorophyll content, net photosynthetic rate and Rubisco activity in flag leaf after full heading stage of five varieties with different yield levels in the first accumulated temperature zone of Hei longjiang province were studied in this experiment. The results show that the chlorophyll content, net photosynthetic rate and Rubisco activity in flag leaf of varieties of high yield were higher than the varieties of low yield and had great difference in these varieties. So research the changes of chlorophyll content, net photosynthetic rate and Rubisco activity in flag leaf after full heading stage of different varieties as a measure in breeding of high photosynthetic efficiency and super high yield of rice is useful in cold region.