A simulation technique has been developed for describing the synchronization mechanism of a transmission for heavy-duty trucks, to clarify why and how abnormal (two-stage) shift reaction force while upshifting occurs sometimes. It is hard to analyze this phenomenon with a conventional experimental approach because the synchronizer mechanism is complex and synchronization performs in a short time. Hence, the movement and contact force of the synchronizer components are simulated with an analytical model using ADAMS. According to the simulation results, the abnormal shift reaction force occurs during the meshing process of both splines of the sleeve and clutch gear, its cause is a backward movement of the sleeve. The relative relationship between a sleeve movement force at the contact point of both spline chamfers and clutch gear rotational force influences the phenomenon, and it can be reduced by tuning the friction force on the chamfer surfaces.