In this study, we attempted to understand changes in the vascular plants of Isl. Pengchiayu, an islet in waters north of the main island of Taiwan. In addition to visiting the islet 3 times to survey the flora, documents regarding floral studies of Isl. Pengchiayu since 1906 and specimens of this islet deposited in 2 local herbaria, TAI and TAIF, were all referenced. Records of species on this islet were divided into 2 periods: the past (1904~1933) and recent (1992~2005) across about 100 years. All the vascular plants known to have existed during this period are listed. Results show that there were 144 species, including infraspecies, inhabiting the islet in the past period. Among them, 53 species have not been found recently. On the other hand, 39 species which apparently did not occur in the past have recently been recorded. In total, 130 species are known to exist on the islets recently. Possible reasons for the changes are also proposed.