Free surface flow simulations using CFD (Computational Fluid Dynamics) can be categorized into two approaches in terms of free surface modeling. One is a two-phase flow model which solves both water and air. The other is a single-phase approach in which only the water region is solved with free surface boundary conditions. Breaking waves have been simulated by many researchers using CFD techniques with two-phase flow models in the most cases. In ship flow CFD applications, however, a single-phase approach is often used since this is considered to be more effective and efficient. However, when a single-phase method is applied to flows with bow wave breaking of a blunt ship, it is observed in some cases that numerical solutions fail to reproduce wave breaking well. This may be due to the differences of the air-water interface treatment between two-phase and single-phase approaches. To investigate these differences, breaking of Stokes waves is simulated by using both single-phase and two-phase flow models. The comparisons of single-phase and two-phase approaches are made in wave profiles, pressure and vorticity distributions. The velocity distributions near the free surface boundaries are also compared. Through these comparisons, impact of free surface boundary conditions in a single-phase approach to wave breaking phenomena is discussed.