The effects of gas composition on the productivity of biomass and intra-cellular fatty acids were investigated in semi-batch cultures of a green alga Chlorococcum littorale (C. littorale) with gas flowing at 100 ml/min under the conditions of temperature of 22 °C and light intensity of 4200 Ix. Growth rates were enhanced and total fatty acids contents increased by flowing an O2-free gas mixture of 2% CO2 and 98% N2. This suggested that the removal of dissolved oxygen lead to promotions of both growth and fatty acids production. It was also found that fatty acids tended to be accumulated under nitrogen deficient conditions in the culture medium based on artificial sea water. Under these photoautotrophic conditions without oxygen in the gas mixtures, the productivity of fatty acids by photosynthesis were more than the oil palm seed used as a main resource of biodeisel, which implies the feasibility of algal oil for biodiesel.