Characteristics of biological CO2 fixation by Chlorella sp. HA-1 were investigated in a semi-continuous and series reactor system using an internally illuminated photobioreactor to overcome shortcomings of physicochemical technologies such as adsorption and membrane separation. High CO2 fixation rate was achieved in the semi-continuous reactor system, in which the dilution ratios of the culture medium were controlled. The average CO2 fixation rate was maintained almost constantly when the dilution ratio increased by 0.1 increment from the initial value of 0.5. The total removal efficiency of CO2 was enhanced by employing a series reactor system. The average CO2 fixation rate increased until 4.013 g CO2 day(-1) in a series operation of four reactors, compared to 0.986 g CO2 day(-1) in a batch operation mode. The total CO2 fixation rate was proportional to the number of reactors used in the series reactor system. In the series reactor system of semi-continuous operation. a large amount of CO2 was removed continuously for 30 days. These results showed that the present reactor systems are efficient and economically feasible for a biological CO2 fixation.