In order to efficiently provide IP based Wireless Broadband services in Mobile WiMAX, measures for more efficiently utilizing a limited bandwidth in radio links are needed. Robust Header Compression (ROHC), a standard header compression scheme proposed by IETF, Bidirectional Optimistic (0) mode provides higher compression efficiency and robustness compared with Payload Header Suppression (PHS) which is an optional header compression scheme for mobile WiMAX . However, if consecutive packet losses occur because of a Shadow Region of Mobile WiMAX, header decompression failure rate of the ROHC 0 mode increases. Therefore, a complementary mechanism is needed. In this paper, we propose an approach for the dynamic adjustment of an optimistic parameter, which has an effect on both the compression efficiency and the robustness, for improving the performance of ROHC 0 mode. We also analyze the performance of the proposed approach using an OPNET simulator.