The microalga incorporated photobioreactor is a highly efficient biological system for converting CO₂ into biomass. Using microalgal photobioreactor as CO₂ mitigation system is a practical approach for elimination of waste gas from the CO₂ emission. In this study, the marine microalga Chlorella sp. was cultured in a photobioreactor to assess biomass, lipid productivity and CO₂ reduction. We also determined the effects of cell density and CO₂ concentration on the growth of Chlorella sp. During an 8-day interval cultures in the semicontinuous cultivation, the specific growth rate and biomass of Chlorella sp. cultures in the conditions aerated 2-15% CO₂ were 0.58-0.66d⁻¹ and 0.76-0.87gL⁻¹, respectively. At CO₂ concentrations of 2%, 5%, 10% and 15%, the rate of CO₂ reduction was 0.261, 0.316, 0.466 and 0.573gh⁻¹, and efficiency of CO₂ removal was 58%, 27%, 20% and 16%, respectively. The efficiency of CO₂ removal was similar in the single photobioreactor and in the six-parallel photobioreactor. However, CO₂ reduction, production of biomass, and production of lipid were six times greater in the six-parallel photobioreactor than those in the single photobioreactor. In conclusion, inhibition of microalgal growth cultured in the system with high CO₂ (10-15%) aeration could be overcome via a high-density culture of microalgal inoculum that was adapted to 2% CO₂. Moreover, biological reduction of CO₂ in the established system could be parallely increased using the photobioreactor consisting of multiple units.