Vertical axis wind turbines (VAWTs) have attracted attention due to its high productivity and low noise generation, compared to horizontal axis wind turbines (HAWTs). Considering the various designs and the wide operating range of Reynolds number, computational fluid dynamics (CFD) is indispensible for analyzing VAWTs. In this paper, an unsteady flow simulation using an immersed boundary method (IBM) for a VAWT was performed. A non-physical spurious force oscillation (SFO) phenomenon due to the nature of IBM was found during the computation of the torque coefficient of the VAWT, and statistical analysis showed that SFO could be converged to zero after time-averaging. Ultimately, the performance coefficient of the VAWT was calculated. Compared to the reference data, the computed data showed the same tendency in all tip speed ratio (TSR) regions between 1.6 to 2.4.