Xiyu Township is composed of Yuweng Island and Xiaomenyu; and as Xiyu is located west of the Penghu Islands, it is known as Xiyu (or West Isle). Xiyu is the largest island besides the main island of Penghu and with a total area of 18.715km2. As cold and warm ocean currents converge here, plankton in the ocean are especially abundant, and large numbers of fish are able to inhabit here, which have made Xiyu the township in Penghu with the greatest yield of catch.