Abstract: In communication systems, the application of microstrip active antenna is useful for the simplification and downsizing of the system. But because the antenna is integrated with active devices, the harmonics produced by the active devices can result in the spurious radiation of the antenna. In this paper, the application of PBG(Photonic Band-Gap) structure to the active antenna is studied, and the calculated result and experimental result all indicate that the application of PBG structure can drastically diminish the radiation of harmonics, so the performance of system is improved.