A finite difference numerical model for tidal current for the region of wide open boundaries has been established. A special attention is paid to developing a new method to treat open boundary condition, Along the open boundaries, it is supposed to know the sea surface elevations. Therefore, it is possible to apply the continuity equation to find boundary velocity at each boundary point using the elevations and the interior velocities adjacent to the boundary point at every time step. After the velocities along the boundaries are calculated, they are used to find a solution for the interior region. At the next time step new boundary velocities are calculated using the solution. The whole process is repeated until the solution converses to the tolerable range. This model with 4 tidal components, N₂, S₂, K₁, and O₁, is applied to the sea off Kunsan, the west coast of Korea. The model converses after about a-day's run when the time difference at each step is 30 seconds. The velocities produced by the model with the developed boundary method show no instabilities in the domain of computation. In comparision with the observed velocities, the computed ones show a fair agreement.