In order to evaluate the effect of different levels of defoliation and grain removal on grain yield and photosynthesis rate of sunflower, a factorial experiment was conducted based on randomized complete block design with three replications during 2012 growing season at College of Agriculture, Shiraz University. Treatments were defoliation at 3 levels (0, 50, and 75%) in heading stage and grain removal (without grain removal (control), removal of one half of the grains in inner or side parts) at pollination stage. It appeared that defoliation resulted in reduction of grain yield, 1000 grain weight and grain number. Apparently increased photosynthetic rates of reminder leaves may be considered a response to the reduction of leaf area however it did not generally sufficient to compensate the reduction of leaf area. The highest reduction of grain yield was recorded in 75% defoliation and removal of one-half of the grains in inner parts of head, compared to control. It can be concluded that grain yield of sunflower is more determined by limited source.