The concentration of carbon dioxide in natural seawater is sufficient for microalgal growth, but insufficient for high algal-density culture due to limitation of photosynthesis in the artificial medium. This study was adjusted the initial pH to 5.5, 6.5 and 7.5 by adding CO2 or HCl in 1L flask cultured stagnantly with continuous illumination. The best growth of Isochrysis galbana was found in the culture (CO2-6.5i) with initial pH 6.5 using CO2 adjustment and maintaining the same during day 2 to day 5, while the worst was in CO2-5.5i. Furthermost, we adjusted the pH of cultures daily to the set values; the best growth was also found in CO2-6.5e having the cell number 221% of the blank. Initial addition of NaHCO3 with doses of 0.01 g/L, 0.05 g/L, 0.1 g/L, 0.5 g/ L and 1g/ L in the culture, showed the lowest cell number after 5 days culture is in the group of 1.0 g/L and no significant difference among the rest groups. Addition of 0.5 g/L NaHCO3 and adjusted the pH to 6.5 by HCl in the beginning promoted algal growth and resulted in the culture having 212 % cell number of the blank. Using the feedback control system, 100 L algal cultures with aeration and providing CO2 gas or HCl liquid to maintain the pH as 5.5, 6.5 and 7.5 or 7.5 individually were conducted to test the effect of pH control on the mass culture of I. galbana. Better growth was found in the culture with CO2 feedback control than HCl-control in duplicate experiments. It also showed significant difference among the groups adjusted pH between 6.5-7.5. The cell concentration could reach 1100-1400 x 104 cells/ml and was about double the amount of the blank without pH control cultured in 7 days. Meanwhile, the NO3-N concentration was nearly exhausted while the PO4-P still replete. This study reveals the high concentration and fast growth of I. galbana can be maintained under the suitable physical condition providing the carbon source in an optimal pH.