Abstract: In order to meet some specific application requirements for microstrip patch antenna in navigation and communication project, a novel defected electromagnetic band-gap (EBG) and ground structure designed for improving the performance of traditional microstrip patch antenna was presented. For building this structure, several EBG elements were firstly placed near a usual circularly polarized square microstrip patch with two opposite corners cut. This can enhance the resonance frequency and change only slightly the bandwidth of the patch antenna. Then a thin square metal layer was cut off from four sides of the ground plane to forms a square loop of dielectric layer. The results of numerical simulation show that the new structure is able to improve the performance of antenna gain including total and low elevation situation, and the microstrip patch antenna equipped with this structure can be treated as a dual-frequency antenna.