UHV precision mono-beam slits are being designed and used at Taiwan Photon Source (TPS) beamlines in the National Synchrotron Radiation Research Center (NSRRC). The slits serve to shape the beam sizes and simultaneously to act as monitors of the position of the photon beam (PBPM). The main features of the slit system are the compact and robust chamber to accommodate the slit components, the fine-adjustment mechanisms to tune slit-blade parallelism and the multi-flange bellows for ease of assembly of the entire system. The slit opening is adjustable from 0 to 40 mm both vertically and horizontally. This paper presents the design features of the slit system and the commissioning result of the BPM function.