Clostridium butyricum was incubated in 10m mol, 30m mol 50m mol and 70m mol phosphate buffer medium (pH 7.0). With the high concentration buffer, the pH value decreased more slowly. The H_2 evolution hydrogenase kept on high activity level for a longer time, While the H_2 uptake hydrogenase kept constant. So more hydrogen was produced. The hydrogen production nearly doubled in 70m mol phosphate buffer than in 10m tool phosphate buffer. The result shows that to keep pH constant can increase the H_2 production of C. butyricum.