The application of photonic bandgap structures (PBG) as substrates in microstrip circuits has been investigated.The characteristic of stopband and effect to microstrip circuits are introduced.Novel cascade and hybrid parallel PBG structures based on ground defected structures are presented as inner microwave broadband filters.Experiments have been done and the frequency transmission characteristic is measured.Existence of the frequency bandgap and feasibility to construct microwave broadband filters have been proved.