Due to the significant oxygen requirement during citric acid production and the relatively low solubility of oxygen in water, aeration is critical. The potential use of n-dodecane as an oxygen-vector for improvement of citric acid production by Aspergillus niger was studied. The volumetric fraction of oxygen-vector has a great influence on the volumetric oxygen transfer coefficient k L a. With the addition of an oxygen-vector to the fermentation medium with a final concentration of 5%, the k L a value reached a maximum value (130 h - 1 ), which is twice that of the control experiment. The addition of 5% (v/v) n-dodecane enhanced citric acid accumulation, reduced residual sugar concentration and stimulated mycelial growth. Adding n-dodecane had no adverse effects on the cells of A. niger. The results of enzyme assays indicated that no significant differences were observed between the activity of citrate synthase of two kinds of mycelial cell-free extracts.