In this thesis a filter for a duplexer system is designed and simulated. The most important factor of the design was the robustness against fabrication imperfections and the fast and low-cost manufacturing. In the first part of the thesis the basic concepts of filter design are discussed and mathematical models of filters are shown. In the following part the construction of a duplexer and its parts, especially waveguide filter types are presented. In the last part, the filters are designed and the responses are simulated. After the optimization of the filters, the fabrication tolerances are investigated. The responses versus the imperfections and thermal expansions are presented. The finline filter has Chebysev response, the corrugated tapered filter has quasi-Zolotarev characteristics, these have been proven to be optimal to satisfy the wide-band requirements. The thesis was supported by the Ericsson - Budapest University of Technology and Economics Department of Broadband Infocommunications and Electromagnetic Theory.