Abstract: Carbon-nitrogen ratio (C/N) is one of the inhibitive factors to straw anaerobic fermentation. To study the appropriate C/N of corn straw, bacterial protein and urea were added into the fermentation broth until the C/N reached 25/1 and 35/1, respectively. The whole fermentation experiment was carried out for 30 days, and the results indicated that the additions of bacterial protein and urea accelerated the degradation of straw and inhibited the straw volume expansion. Under the condition of the same load of straw, the test group which C/N was 35/1 degraded straw with faster rate than the group which C/N was 25/1. Compared with R0 (no additive), the gas yield of R2 (adding bacterial protein) and R4 (adding urea) which C/N was 35/1 increased by 31.43% and 2.69%, respectively. Furthermore, the addition of bacterial protein and urea displayed acid and alkaline buffer action to fermentation broth.