This paper represents a new powertrain simulation methodology using the object-oriented programming paradigm. The advantage of the object-oriented is the module interchangeability and simulation flexibility. Powertrain subsystems and controller modules are implemented using the MEX files in MATLAB Simulink in this paper, preserving module interchangeability. Currently, the required CPU time on a 75MHz Pentium PC is about three times the real time. It is anticipated that the "Automotive Powersystem Toolbox" being developed in this research would be of much utility in designing subsystem controllers as well as in designing subsystem mechanicals.chanicals.