Selecting a high-quality site for wind turbine installation is frequently challenging in Japan because the majority of the topography of Japan is steep complex terrain and the spatial distribution of the wind speed is quite complex over such terrain. To address this issue, an unsteady CFD model called RIAM-COMPACT【○!R】 has been developed. RIAM-COMPACT【○!R】is based on an LES turbulence model. In the present paper, to assess the accuracy of RIAM-COMPACT【○!R】, we performed a numerical simulation of uniform, non-stratified airflow past a three-dimensional isolated-hill. Attention is focused on airflow characteristics in the wake region. The results of RIAM-COMPACT【○!R】 using the LES model were in good agreement with those of another commercially-available CFD software (STAR-CCM+) using LES models.