Three-dimensional transient numerical simulation of gas exchange process in a four-stroke motorcycle engine with a semi-spherical combustion chamber with two tilt valves was studied. Combination of the grid re-meshing method and the snapper technique made the valves move smoothly. The flow structure and pattern in a complete engine cycle were described in detail. Tumble ratios around the x-axis and y-axis were analyzed. Comparison of computed pressure with experimental pressure under motored condition revealed that the simulation had high calculation precision; CFD simulation can be regarded as an im-portant tool for resolving the complex aerodynamic behavior in motorcycle engines. Three-dimensional transient numerical simulation of gas exchange process in a four-stroke motorcycle engine with a semi-spherical combustion chamber with two tilt valves was studied. The combination of the grid re-meshing method and the snapper technique made the valves move smoothly. The Tumble ratios around the x-axis and y-axis were analyzed. Comparison of computed pressure with experimental pressure under motored condition revealed that the simulation had high evaluation precision; CFD simulation can be considered as an im-portant tool for resolving the complex aerodynamic behavior in motorcycle engines.