Abstract: Four Simmental steers (average BW 420 kg, aged 2.5 years) cannulated in rumen consuming corn straw diet were used in 4×4 Latin square design to evaluate the effects of different selenium methionine on ruminal pH, ammonia, volatile fatty acid, nutrient degradability and purine derivative of urine. Treatments consisted of 0 mg (control), 0.3 mg, 0.6 mg and 0.9 mg supplemental Se (selenium methionine) per kg DM diet. The results show that ruminal pH, volatile fatty acid concentration and actate to propionate ratio are not affected (P> 0.05). Ammonia concentrations in supplemented selenium methionine groups are lower than the control (P< 0.05). Degradability of dry matter, organic matter, and neutral and acid detergent fiber of corn straw, and the degradability of dry matter, organic matter and crude protein of soybean in the 0.6 mg supplemental Se per kg DM are significantly higher than the control group (P< 0.05). Allantoin and purine derivatives in group supplemented with 0.6 mg and 0.9 mg Se per kg are significantly higher than both the control and the 0.3 mg supplemental Se per kg DM (P< 0.05). Supplementation of selenium methionine has passive effect on rumen fermentation. Optimum Se supplementation dose is 0.6 mg·kg^-1.