Abstract: The effect of heat stress in the grain-growing period of wheat was studied.The results show that post-anthesis high temperature influences the grain yield and the quality of wheat by altering the source and sink functions.The effects of high temperature on the grain growth depend on the timing of heat exposure and the degree of heat stress.Under the moderately high temperature （25～32℃）,the grain mass primar ily reduces by reducing the duration of grain filling.Under the very high temper ature （33～40℃），the filling rate decreases and the sustainable period of fil ling shortens,finally the grain weight decreases significantly.Soluble starch sy nthase（SSS）is very sensitive to high temperature.It is shown that the reduction in the rate of grain growth at high temperature is mainly due to the reduced a ctivity of SSS.Compared with leaf blades,the non-leaf green or gans in wheat possess higher levels of C₄ photosynthetic enzymes and have greater heat tolerance in photosynthesis.Therefore,in order to improve the grain yie ld of wheat under high temperature conditions,it is important to strengthen the role of non-leaf organs in photosynthetic source and inc rease the activity of SSS in grain sink.