Abstract: The effects of temperature,light intensity,pH value and salinity on photosynthesis of Botryococcus braunii UTEX 572 and B.braunii UTEX 2441 were studied by measuring net photosynthetic O₂ evolution.For B.braunii UTEX 572,the suitable light intensity range was 400-1600 μmol·m^-2·s^-1,with the saturation light intensity 800 μmol·m^-2·s^-1;the suitable temperature range was 25-35℃,with the optimal temperature about 30℃;the suitable pH range was 5.0-8.0,with the optimal pH about 7.0,the suitable salinity range was 0-0.2 mol·L^-1,with the optimal salinity about 0.1 mol·L^-1.For B.braunii UTEX 2441,the suitable light intensity range was 400-1600 μmol·m^-2·s^-1,with the saturation light intensity 400 μmol·m^-2·s^-1;the suitable temperature range was 25-35℃,with the optimal temperature about 30℃;the suitable pH range was 5.0-8.0,with the optimal pH about 7.0;the net photosynthetic O₂ evolution decreased dramatically while the salinity increased.The photosynthetic characteristics of two B.braunii strains indicated the basic physiological and ecological characteristics:adaption to relative strong light intensity,relative high temperature,neutral to acid environment and lower salinity.The results are helpful for optimizing the culture condition.Some differences exist between two B.braunii strains in their response to the change of light intensity,temperature,pH value and salinity,compared with B.braunii UTEX 2441,B.braunii UTEX 572 has higher saturation light intensity,adaptive to higher temperature and much broader pH,its growth is promoted by mild increase of salinity,which indicates that B.braunii UTEX 572 has greater potential for fast growth in cultivation,the result provided evidence for selecting good strain that suitable for mass culture of B.braunii.