We report on what we believe to be the first truly distributed birefringence measurement of polarization-maintaining fibers (PMFs) based on transient Brillouin grating (TBG). A TBG is created by two short pump pulses in the slow axis of the PMF, and then the birefringence-related TBG spectrum is mapped by scanning a probe pulse launched in the fast axis, where the local birefringence can be calculated using the birefringence induced frequency shift. Two types of widely used PMFs, bow-tie and panda, with a length of 8 m were measured at a spatial resolution of 20 cm, and the results show that the birefringence features a periodic variation, and their variation ranges are approximately 2.4x10(-6) and 1.3x10(-6) along the test fibers, respectively.