A xantha rice mutant was used as the material,which was subjected to high temperature stress treatment (9:30～17:30,40℃；the temperature of other time period was the same as natural),to study the impact of photosynthetic characteristics and chloroplast ultrastructure on its flag leaves in high-temperature stress.The results showed:during the high temperature stress period,the flag leaves net photosynthetic rate(P_n),PSⅡ primary photochemical efficiency(F_v/F_m),PSⅡ optical quantum efficiency(ф_PSⅡ),acyclic photosynthetic electron transfer rate(ETR),the content of chlorophyll and soluble protein decreased,but initial fluorescence(F₀) and Membrane permeability increased.Grana lamellae structure of chloroplast in blades was rough and disordered,the membrane number of grana lamellae structure was relatively less,matrix fades.All above indicated that high temperature stress caused destruction of chloroplast ultrastructure in xantha rice mutant and decrease of blades photosynthetic capacity.