Abstract: Equilibrium pH value, one of the important chemical parameter of medium, is the pH value when CO2 partial pressure in the micro alga medium equals to CO2 partial pressure in the air. The three mostly used medium (BG11 medium, BBM medium and Zarrouk medium) were used to study the effects of carbon, nitrogen and phosphorus concentration and salinity on the equilibrium pH value by air bubbling aeration of the medium. Our results demonstrated that the increased NaHCO3 concentration in the range of 016.8 g/L enhanced the equilibrium pH values of all three medium. In the concentration range of 02.5 g/L, NaNO3 had no significant effect on equilibrium pH value of these three medium. 0.1 g/L KH2PO4 raised the equilibrium pH value of BG11 medium, but decreased it in BBM medium. The further increased P concentration (maximum 0.4 g/L) did not regulate the equilibrium pH value of both BG11 and BBM medium. The P concentration in the range of 00.4 g/L did not affect the equilibrium pH value of Zarrouk medium. The salinity in the range of 034 decreased the Equilibrium pH value of all three medium. Carbon concentration is the most important factor to influence the equilibrium pH value of micro algae medium, and the relationship between equilibrium pH value and carbon concentration can be described with the regression equation: y=0.3504ln(x)+8.9647 (R2=0.9708). N concentration had no significant effect on equilibrium pH value. P concentration and salinity had limited impact on equilibrium pH value. The efficiency of CO2 utilization will be improved at higher pH value compared with the equilibrium pH value. These results support the environmental benefit of biofixation of CO2 by micro algae.