Abstract: Four Simmental steers(average BW 420±8.6kg, aged 2.5) with permanent rumen cannula and consuming a corn straw diet were used in a 4×4 Latin square design and supplemented with four levels(0, 0.02, 0.04 and 0.06g/kgW) of supplemental isovalerate to evaluate the effects of isovalerate on ruminal pH, NH3-N, VFA, nutrients effective degradability and purine derivatives of urine. The results showed that ruminal pH in the steers given supplemental isovalerate 0.06g/kgW decreased (P<0.05). The NH3-N concentrations in the rumen of the animals in 0.04g/kgW and 0.06g/kgW group were lower than control and in 0.02g/kgW group significantly (P<0.05). Soybean DM, OM and CP effective degradability in the rumen of the animals given isovalerate supplementation with 0.04g/kgW and 0.06g/kgW were lower than control. The effective degradability of DM, OM, NDF and ADF of corn straw in the rumen of the animals given isovalerate supplementation with 0.04 g/kgW were increased significantly (P<0.05). Ruminal actate, butyrate, acetate/propionate, TVFA, and purine derivatives in urine of the animals given isovalerate supplementation with 0.04g/kgW were increased significantly (P<0.05) . These results indicated that the optimum dose of isovalerate supplementation was 0.04g/kgW.