A novel diplexer design by combining of three electromagnetic band gap (EBG) structures is presented in this paper. The diplexer is constructed by combining an EBG at the input port with two different EBG at the output ports. The proposed diplexer is configured with filter in each arm to reject unwanted signals. The first or the second bandpass falls into the stop- band and can be suppressed by EBG structures as well as band pass is controlled directly by EBG structure. Further the EBG structure demonstrates a good performance to improve the mutual coupling. Practical applications of EBG structure usually have difficulty in accommodating its physical size, because the period of EBG lattices has to be a half-waveleng that the band-gap frequency. The proposed design has been implemented in a 25 mil thick Rogers RO301 0substrate. It provides a new idea for designing more efficient structures for microwave circuit applications, such as filters, diplexer, and other passive circuits. This research is only the first step for the feasibility study of the diplexer design by using EBG technology, which can be modified to the other desired frequency bands. The proposed diplexer is simple to fabricate, as it is designed using microstrip technology, and presents compact dimensions, making use of a low number of elements. The excellent simulation results of transmission, reflexion and isolation magnitudes are an excellent starting point to develop a robust design methodology.