In this study three species of microalgae Isochrysis galbana, Chlorella sp., Nannochloropsis oculata for biomass culture were cultured for biomass in cement tanks (2000 litres per tank) in Qutdoor in order to determine the daily harvest ratios and the optimal cell density. For each species of microalge, the experiment consisted of 3 different treatments of daily harvest ratios: 10 percent, 20 percent and 30 percent. F/2 medium was used. The results showed that, the treatment 20 percent with the highest cell density in, three species of microalgae Nannochloropsis oculata, Isochrysis galbana and Chlorella sp. was 24.65 percent 0.31 x 10 exponent 6 cell/ml, 15,84 percent 0.52 x 10 exponent 6 cell/ml and 18,62 percent 0.15 x 10 exponent 6 cell/ml, respectively.