In the development of an air-cooled engine, cooling air flow was analyzed by CFD. Cooling performance of the engine is necessary for precise temperature control. However, stable radiation is difficult for motorcycles because of the effects of driving wind. Cooling fins are thin and complicated; therefore plenty of time and effort is necessary to create a boundary fitted mesh. In addition, since low Reynolds (Re) number areas coexists with high Re number areas in a calculation domain, applications of the wall function are limited. Therefore, we employ the Partial Cells in Cartesian coordinate method, which expresses complex shapes according to turbulence boundary conditions in the Cartesian coordinate. Analysis results show good agreement with test results in a wind tunnel. Furthermore, useful knowledge in engine development was obtained by examination of local heat transfer coefficients calculated by Karman's analogy.