The purpose of this study is to apply the Penman-Monteith equation to estimate potential evapotranspiration occurred in Taiwan, an equation not only widely used in the world but also officially recommended by FAO. In addition, the Penman-Monteith equation is considerably more complex than other equations, which has been also used extensively in water budget. In this study, various empirical equations from different theoretical models for calculating potential evapotranspiration are compared with the Penman-Monteith equation based on daily meteorological data from four meteorological stations in Taiwan. Monthly potential evapotranspiration was estimated using equations developed by Blancy-Criddle (1950), Hargreaves-Samani (1985), Kharrufa (1985), Makkink (1957), Priestley-Taylor (1972), and Hargreaves (1975). Finally, three statistical criterions are used to establish the optimal parameter suitable to the local hydrological condition of Taiwan.