Gas-liquid stirred tanks are widely used in various chemical engineering processes such as fermentation and pharmaceutical production. In the mixing process for fermentation, aerobic microbes produce high polymer compounds from oxygen and carbon sources which need to be supplied by agitation and aeration in a tank. It is essential to supply a sufficient and uniform amount of nutrients to provide high quality and quantity products, and hence, it is necessary to optimize the shape and operating conditions of the stirring tank to mix gas and materials in the liquid sufficiently and uniformly. However, it is difficult to obtain a guideline for the design of plant-scale tanks from laboratory-scale experiments because experimental measurements cannot provide the detailed distribution of materials. The present study aims to obtain a guideline for an optimized design by developing computational fluid dynamics (CFD) simulation of the gas-liquid two-phase flow in the stirred tank and investigating the mass transfer inside the stirred tank in detail. Applying k-ω shear stress transport (SST) model, we calculate the gas-liquid flow in the stirred tank and estimate overall gas hold-up in comparison with experimental data.