Abstract: The two-step fermentation process was studied for preparing 2-keto-l-gulonic acid, the precursor of vitamin C, from sorbitol in an annulus airlift fermentor (AALF) of 400 mm diameter.The oxygen transfer rate and volumetric mass transfer coefficient of the second step of the fermentation process were measured. The results show that the average volumetric mass transfer coefficients in middle stage of fermentation are onlg 40% of those in the air-water system at the same superficial gas velocity. A performance index for evaluating operation performance of fermentors was proposed.The optimum air flow rate of AALF was obtained, and it compared with the results of stirred-tank fermentor (STF). The results also show that AALF is a very suitable formentor for the two-step fermentation process.Under proper operation conditions, its fermentation yield is 85%.Therefore AALF can completely replace STF in industrial production, and greatly reduce power consumption and investment cost.