Sixteen cows were divided into control and trial group according to age, parity, calving days and milk production. Cows were supplemented with 12 g/d rumen protected methionine (Met) in the trial group. Effects of rumen protected Met on performance and the apoptotic rate of blood lymphocytes and expression of apoptosis related genes in dairy cows in summer were studied. The apoptotic rate of lymphocytes was analyzed by flow-cytometry. The gene abundance of Bcl-2 mRNA and Bax mRNA were analyzed by quantitative RT-PCR in the blood lymphocytes. The results showed that supplementing 12 g/d rumen protected Met could improve tendency of milk yield, mild fat, milk protein, lactose and non-fat solid content. Somatic cell had the decreased tendency when supplementing 12 g/d rumen protected Met. Supplementing 12 g/d rumen protected Met could decrease the level of glutamate pyruvate transaminase (GPT), glutamic oxalacetic transaminase (GOT) and creatine phosphokinaxse (CPK) of plasma compared with control group but there were no significant differences in these indices (P>0.05). The level of LDH of plasma decreased by 10.37% than that in control group and the difference was significant (P<0.05). The levels of ALP, GLU, SOD and GSH-Px were improved while MDA was decreased than that in control group but the differences were not significant (P>0.05). The apoptotic rate of lymphocytes decreased by 36.39% in supplementing 12 g/d rumen protected Met group than that in control group and the difference was significant (P<0.01). The abundance of Bcl-2 mRNA of lymphocytes was improved by 11.92% and the abundance of Bax mRNA was decreased by 17.30% in trial group than that of control group. The ratio of Bcl-2 mRNA to Bax mRNA was increased by 36.36% in trial group than that in control group but the difference was not significant (P>0.05). It was concluded that supplementing 12 g/d rumen protected Met could improve performance, scavenge free radicals, decrease apoptotic ratio of lymphocytes, control expression of apoptosis related genes and moderate heat stress in dairy cows in summer. ［Chinese Journal of Animal Nutrition, ２００9，２1（5）:665-672］