Abstract In this paper, a new microstrip diplexer using modified H-shaped resonators with a compact size and high isolation is presented. This circuit is designed for the ISM band at 2.4 GHz and GPS at 1.575 GHz. The proposed diplexer is formed by connecting two band-pass filters designed by using the modified H-shaped resonator and tapped microstrip Input/output (I/O) lines. An analytic study of the H-shaped resonator and an equivalent LC model of the proposed H-shaped band-pass filter were introduced. Two cross-slots are etched in the ground plane of the proposed microstrip diplexer in order to enhance its electrical performances and to set the pass bands at low frequencies. The circuit with defected ground structure is able to operate at the GPS L1 and L2 bands. The electrical performances of the diplexer are investigated numerically by using Momentum integrated in ADS from Agilent technologies and verified with another electromagnetic software CST microwave Studio.