This paper presents the features and differences of NS2 and OPNET by comparing them in the aspects of software technique, operation method and engineer performance. As to the software technique, it includes four points for comparison: system architecture, supported network technique, network equipment model bank and software scalability. To compare the performance of the simulator engine, an abstract network which is not real was established and simulated by these two simulators separately under identical conditions. Meanwhile, two performance indices (memory consumption and computation time) were measured, recorded and analyzed. The comparison result indicates that OPENT as a commercial software is better than NS2 in user-friendliness, simulation speed and memory consumption while NS2 is superior to OPNET in the aspect of open-source and scalability and well suited for small scale network simulation.