This study aimed to investigate the symbiotic relationship of two freshwater microalgae species, Ettlia sp. and Chlorella sp. HS-2 to improve biomass productivity. The species were co-cultivated autotrophically under the inoculation ratios (Chlorella/Etdia) of 1:01, 1:04, 1:08, and 1:16. The performance of co-cultivation was compared with monoculture. It found that the co-cultivation returned higher biomass productivity (P < 0.05) than the monoculture under all inoculation ratios. The highest biomass productivity of 0.70 +/- 0.02 g L-1 day(-1) was achieved with an inoculation ratio of 1:08. The biomass productivity further increased to 0.74 +/- 0.06 g L-1 day(-1) by switching the cultivation mode to the mixotrophic condition. The biomass pro- ductivity of mixotrophic co-cultivation was higher P < 0.05 (0.74 +/- 0.06 g L-1 day(-1)) than the mixotrophic mono-culture of Ettlia (0.41 +/- 0.06 g L-1 day(-1)) but equal to mixotrophic mono-culture of Chlorella. FACS analysis revealed that the biomass obtained after co-cultivation contained 81% of Chlorella cells. In biochemical composition, co-cultivation (autotrophic) biomass contained protein 41%, carbohydrates 33%, lipids 11%, and pigments 2% of dry cell weight. The major fatty acids produced in the co-cultivation were C16-C18, which are favorable for biodiesel production. It is concluded that the co-cultivation is more favorable than the monoculture to obtain high biomass productivity and stable biomass composition.