The design of a 4-channel multiplexer using a waveguide cylindrical cavity resonator of second order and fourth order is presented in this paper. The performance of the multiplexer is improved when implementing the higher order. The proposed multiplexer design is based on a manifold structure that uses a TE011 mode resonator as the filter channel element. The design procedure involves the use of FD3D and FEST 3D of CST Microwave Studio software to optimize the dimensions of the resonator and waveguide sections to achieve the desired performance parameters, such as insertion loss, return loss, and isolation. The simulated results show that the proposed multiplexer design achieves good performance by featuring the insertion loss is less than 1 dB, and the return loss is better than 10 dB over the 10 GHz band. The isolation between the channels is more than 10 dB, which is suitable for most applications. The proposed multiplexer design is simple, compact, and has good performance characteristics, making it suitable for use in various microwave applications. © 2023 IEEE.