Simulation technology has been widely used in all aspects of military applications. In different applications, the fidelity requirements are also different, and so are the simulation models. Using the development of new military aircraft as an example, the simulation fidelity requirement would be very high. In this case, the simulation model has to include many details. On the other hand, the pilot training simulator has to satisfy the real-time simulation requirement; its fidelity cannot be very high and the model has to be simplified. Therefore, the reusability of military platform's simulation models is very low. This paper suggests an Object-Oriented approach to the modeling of military platforms. A helicopter is chosen to be the sample platform. To limit the scope of the problem, only the dynamical model of the helicopter is considered. This model includes equations of motion, kinematics, power plant, and interaction with the environment. The helicopter dynamical models can have many levels of detail. In a constructive simulation, it is possible that only the positions of the helicopter are of interest; therefore, a simple kinematic model may be sufficient. On the other hand, in a wargame simulation, the helicopter responds to control commands and moves from one position to another. A point-mass model can represent such motion. In a helicopter pilot trainer, though, a six-degree-of-freedom model is needed to represent both linear motion and the roll-pitch-yaw orientation. However, the real-time simulation requirement prohibits the model to use sophisticated aerodynamic models. Thus, in some applications, only two-dimensional motion is needed; in other applications, a four-degree-of-freedom model is sufficient. The object-oriented approach uses the concept of hierarchy and inheritance to build classes ofcomponents. Based on the fidelity requirements, classes and sub-classes can be replaced. This approach greatly increases the reusability ofthe model.