Engine start is a very crucial phase in the operation of automotive engines, and the starter motor plays a vital role in this short transient period. However, a complete and exhaustive model of the combined engine-starter system has not appeared to date in the open literature. Although many researchers have modeled the starter system in engine models, the role of the roller one-way clutch is typically omitted. The roller one-way clutch not only protects the starter motor from damage due to high (transient) engine RPM, but it also enables the starter motor to deliver only positive torque to the engine crankshaft. As the trend for model-based calibration grows, the use of computer simulation in engine cold start and crank-to-run transition problems demand high-fidelity simulations and modeling, which in turn requires the inclusion of an accurately modeled oneway clutch mechanism. In this paper, a novel roller one-way clutch model is developed based on the principles of the oneway clutch. Simulation results are presented for a cylinder pressure resolved engine model and results for starter current characteristics and engine RPM, are verified against actual engine data obtained during the starting phase.