The IEEE Standard 802.16 (WiMax) promises to provide wireless broadband access to homes, businesses and core telecommunication networks worldwide. However, security is a key concern to the success of IEEE Standard 802.16. There has been extensive work on improving the security of IEEE Standard 802.16 for both fixed and Mobile networks in PMP mode, but thus far there is scarcity of research on security in IEEE 802.16, and its impact on the performance on such networks. In this Paper, we investigate the impact of security, specifically, IPsec on the performance of the 802.16. We explore and analyze the impact of different implementations of encryption techniques, using the two IPsec transmission modes, Our analysis is based on simulation, using the well-known Qualnet software.