Abstract: The effects of CO2 concentrations on cell growth and vitamin B12 biosynthesis metabolism of Pseudomonas denitrificans were investigated by controlling the inlet gas (containing air and CO2) at the adjusted φCO2 of 0.03%, 3.32%, 8.86% and 13.84%. The fermentation physiological metabolism analysis results showed that hardly any significant influence on cell growth was observed when φCO2 increased to 8.86%. When φCO2=8.86%, the specific glucose consumption rate decreased to 29.95 mg/(g·h), only 77% of that of the control, and the specific vitamin B12 production rate reached 1.75 mg/(L·h), 71.6% higher than that of the control. However, when the inlet φCO2 increased to 13.84%, cell growth, substrate consumption, and vitamin B12 production were greatly inhibited. Therefore, it is important for controlling the φCO2 in industrial vitamin B12 fermentation.