Object-oriented Simulation (OOS) is an important developing trend in the current simulation community. Owing to the macro-structural, hierarchical and modular feature of its modeling method, OOS can improve the encapsulation, flexibility, reusability, extensibility and maintainability of simulation models. It has been put into use in many domains. The authors divide current OOS schools into three categories: OODB based, DEVS based and Actor based, introduce their main thoughts and relative simulation modeling method respectively.