Chlorella vulgaris UTEX259 was cultivated using two different methods of gas supply. In one method the CO2 concentration in bubbled gas was held constant and in the other method it was increased gradually. Algal growth was almost linear after a short period of lag phase in both methods. With the constant CO2 concentration, the CO2 fixation rate in the linear growth phase decreased over 10%(v/v) CO2. while the rate increased up to 6% CO2. However, the rate was enhanced by using the latter incremental increase method especially under a higher concentration of CO2. The maximum rate of CO2 fixation was 52 mg CO2/l . h at 20% CO2 during the gradual increase of CO2 concentration.