The application of object-oriented modeling to develop a powertrain performance simulation program, called O-DYN, is introduced. Powertrain components, such as the engine, transmission, shaft, clutch are modeled as classes which have data and method by using object-oriented modeling methodology. O-DYN, a performance simulation program, based on the object-oriented modeling is made in C++. One powertrain simulation using O-DYN is proposed, and it is expected that the simulation program or individual class constructed in this paper would be useful for automotive engineers to predict the performance of powertrains and to develop a simulation program.