Simple and useful evaluation indexes for the high temperature stability of photosynthesis (HTSP) in different rice varieties was studied for screening and identifying germplasm resources and new variety breeding.The diurnal changes of photosynthesis were tested under high temperature both in natural and illuminating incubator conditions using CIRAS-I portable photosynthesis system.Meanwhile,the genetic characters of HTSP were analyzed using F2 population derived from Texianzhan,a worse HTSP variety,and Shuhui 527,a better HTSP variety,respectively.The results showed that the difference value (D-value) of net photosynthetic rate (Pn) at 09:00 a.m and 13:00 p.m,as well as stomatal conductance (Sc),in flag leaf could be regarded as the evaluation indexes for high temperature stability of photosynthesis (HTSP) in the natural condition.The D-value of Pn of individual plant from F2 population of Shuhui 527/Texianzhan showed a continuous distribution at 09:00 a.m and 13:00 p.m under high temperature.Based on the proportion of D-value of individual plant to population,10%～30% accounted for the maximum part.In addition,the aptness test of D-value distribution curve and theoretic normal distribution of Pn in F2 generation indicated that,the trait of midday depression in photosynthesis was inherited as quantitative character controlled by polygene,other than a simple qualitative character.It was suggested that the identification of and screening HTSP should be carried out in advanced generation populations.