Objective:The effect of branched chain amino acids (BCAA) on exercise capability were investigated. Methods In experiment Ⅰ, 21 male Wistar rats were randomly divided into three groups: control group, swimming training group and swimming training+BCAA group in which the rats were fed with fodder containing BCAA supplement. The rats in the two swimming training groups were forced to swim for one hour everyday. After ten days training, the rats were forced to swim for 6 hrs. The survival rate of rats, the contents of lactic acid in serum, blood urea nitrogen (BUN), and the activity of lactate dehydrogenase (LDH) in serum and skeletal muscle, and the mitochondrial level of lipid peroxidation and membrane fluidity in skeletal muscle were observed. In experiment Ⅱ, 75 mice were randomly divided into three groups and fed the same diet as expeiment Ⅰ. After two weeks, the mice were administered 1.0 mg 15 N Gly and forced to swim. The percentage of 15 N Gly in skeletal muscle protein were determined at 1,2,3,4 h after administration respectivley. Results: It was found that BCAA increased survival rate in swimming rats, decreased serum lactic acid and LDH activity, decreased the mitochondrial LPO in skeletal muscle after swimming, and increased LDH activity and the mitochondrial membrane fluidity in skeletal muscle. BCAA also increased the retention time of 15 N Gly in skeletal muscle protein in mice. Conclusion: BCAA can improve mitochondrial function in skeletal muscle and prevent the onset of fatigue and increase exercise capability.