The gas liquid mass transfer volumetric coefficients K(L)a in several kinds of small size shaking flasks were measured with an optical method based on sulfite oxidation without oxygen concentration probe and liquid sampling. It was found that the liquid film on the inner surface of a flask was one of the very important factors to increase K(L)a. The baffled flask was effective to obtain a larger oxygen transfer rate than the normal flask. A new developed baffled cylindrical vessel based on the baffled small flask has about three times larger K(L)a than the normal vessel.