The effects of various CO2 concentrations in CO2 bioconversion by cultivation of microalga Chlorella vulgaris were investigated using a vertical tubular photobioreactor. The response surface technique with central composite design was applied to model the CO2 biofixation rate, the specific growth rate (SGR), and the biomass productivity of C.vulgaris as function of CO2 concentration and cultivation time. The developed nonlinear model was employed to determine the optimum CO2 concentration in an air-CO2 mixture and the cultivation time for maximum CO2 biofixation, SGR, and microalgae biomass productivity. In addition, a multiple responses optimization method was also applied to determine the maximum CO2 uptake rate, the SGR, and the biomass productivity, simultaneously. The predicted optimum values agreed well with the experimental data.