Recently, the photonic band-gap (PBG) structure has received much interest in mi- crowave and millimeter-wave domain. Applications of PBG in transmission line, microwave com- ponents and antennas have been studied and discussed. Especially the compact PBG structure consisting of small metal pads has been demonstrated to improve the performance of microwave circuits and antenna performance. In this paper, a new uniplanar compact photonic-bandgap (UC-PBG) structure was presented. The proposed PBG structure unit has a two dimensional structure of a rectangle box with two branches intersecting at center. Then this identical unit was connected as a periodical structure. The periodical structure acting as the ground plane of the microstrip transmission line was studied. The transmission line model with the proposed UC-PBG structure ground (TL-UC- PBG) was then studied. An equivalent circuit model of this TL-UC-PBG was suggested to analyze the characteristics of the transmission line with PBG ground. From our study, it was found that this structure has a nice character of low-pass filter. At high-frequency, it showed a broad stop-band.The proposed TL-UC-PBG structure could be used in compact microwave circuit.