As a new non-thermal sterilization technology,supercritical carbon dioxide(SCCO2) draws the attention of the public.But the mechanism of microbial inactivation has not been clearly explained.In this study,the extracellular and intracellular pH of a saline solution of saprophytic yeast treated by SCCO2 was determined.The correlation between pH and death rate under SCCO2 was also been discussed.The results indicated that the extracellular pH declined,but the intracellular pH first decreased,then increased,and declined slightly.The decrease of extracellular pH induced by SCCO2 was not the direct factor of death rate,but helped improve the sterilization effect.The change of intracellular pH induced by SCCO2 was the key factor of cell death.