The mass-culture system of marine microalga is improved by routine culture system in a two floors building. The system consists of a cold water bath that contain hundreds of 2.5 L flasks and more than 8 dozens of 5.2L PE bottles, 2 dozens of 40 L glass tanks that on center table and side frame at second floor. First floor and outdoors have thirty 150 L glass tanks and ten 600 L FRP tanks and 3000L cement tanks, respectively. Each tanks of temporary culture microalga is transferred to larger tanks by siphon downstairs. Microalga cultured on first floor was transfered to outdoors by pumping. The average concentration of microalga stock culture in the cold water bath (12~18°C) increases 16 to 34 days then declines. The highest concentration of this microalga temporary culture in 40L glass tanks is 3×106 cells/mL and its maximum increasing concentration is 2.08×106 cells/mL/day at yield. But mass-culture in 150 L glass tanks, 600 L FRP tanks and 3000 L cement tanks gets their highest mean concentration of 2.63×106, 2.89×106, 1.18×106 cells/mL and with their maximum increasing concentration of 1.95×106, 4.42×106, 1.77×106 cells/mL/day, respectively. Microalga in cement tanks grew slowly and decline, in FRP tanks become bloom sometimes. However, microalga was cultured in 150 L glass tanks indoor grew more stable than other vessel of this system. The seawater is sterilized by chlorine and stocked in two 10 m3 tanks with the chlorine residual below 1.5 ppm. Sodium thiosulfate mixes with nutrients was added to microalga culture tanks before sterile seawater with chlorine residual is filled.