The effect of an oxygen vector on the oxygen transfer rate in air-lift bioreactor was evaluated using the ratio of the volumetric oxygen transfer coefficient to the volumetric fraction of the oxygen vector. When n-dodecane and perfluorocarbon were added at final concentrations of 3% and 2% (v/v), respectively, the oxygen transfer rate reached a maximum value. By addition of 3% (v/v) of n-dodecane to the yeast fermentation, the yeast concentration increased to 26.2 g/l which was 20% higher than in the case of fermentation in the absence of the oxygen vector.