Flows past curved hills have been computed by the numerical method based on the boundary-fitted coordinate and the Cartesian coordinates to investigate the difference and demerits of two different approaches. Calculation results at high Reynolds number laminar flow using central difference scheme for convective terms lead to pressure oscillation when the geometry is approximated by rectangular coordinate and results in stable calculation when the geometry is approximated by boundary-fitted grid or by using upwind scheme. LES calculation results with upwind scheme at higher Re predicts similar trend in both grid systems. For calculations of flow past complex boundary, where generating boundaryfitted grid is almost impossible or difficult, rectangular coordinate approximation can be good alternative.