Abstract: The effects of temperature, light intensity and pH on photosynthesis of Scrippsiella trochoidea and Alexandrium tamarense were investigated. The effects of light-dark cycle on growth rate and biomass were investigated by cultivation of the two algae under different photo phases. These two algae were sensitive to temperature, suitable temperature 17-25℃, and the optimal temperature 20-22℃. They could not grow when the temperature was below 10℃ or above 30℃. The saturation light intensity for S. trochoidea and A. tamarense were 400 μmol·m^-2·s^-1 and 650 μmol·m^-2·s^-1, respectively, both of them were adaptable to high light intensity. These two algae were very sensitive to pH, suitable pH 7.0-9.0, and the optimal pH 7.5-8.0, which were accordant to that of the sea water. The photosynthesis of these two algae were not effective when pH was above 9.5, pH10.0 was deadly to them. Growth rate and biomass increased with the extension of photo phase in certain range.