Abstract: This paper contains the following four main parts.1.A brief survey of the definition of potential evapotranspiration Et,and the relation between the potential evapotranspiration Et and the open water evaporation Eo.2.Historical review of the study of evaporation.3. The factors δ/δ+v and v/δ+v have been considered as "weighingfactors" which specify the efficiency of conversion of net radiation into energy of vaporizing water. It was considered that these weighing factors are function of mean air temperature alone, but recent study indicated that these factors are quite related to altitude. We take these factors as the functions of mean air temperature and the altitude of meteorological stations.4. For the purpose of quick calculation, We provide 6 tables for calculating the Penman estimate of potential evapotranspiration. i. e. 1) table of δ/δ+v and diagram of v/δ+v at different altitudes and for different mean airtemperatues,2) table of conversion from different mean air temperatures and relative humidities to saturation deficit, 3) table of (0.56-0.08√ed),4)table of (0.10+0.90n/N), 5) table of (0.248+0.752n/N, and 6) Table of conversion from U₁₀ to 0.26 (1+9.8×10^-3U₂).