The objective of this study is to improve and modify the evapotranspiration module designed with Penman-Monteith (PM) approach embedded in SWAT2000 distributed hydrological model so as to improve the precision of evapotranspiration (ET) estimation in hydrological process simulation. For PM approach in SWAT2000 ET0 simulation, some of its daily parameters such as air pressure, albedo and soil heat flux were simplified, for overcoming this shortcoming, an improved Weather Generator (WGEN) were proposed. By means of the observed and WGEN simulated meteorological parameters, systematic study on the ET estimations with PM approach for arid, semi-arid Heihe River Basin and humid, precipitation-rich Hanjiang River Basin was conducted. It was found that the computed ET with PM approach fairly correlates with the field observed ones, but biased to be a little lower in quantity than the observed ones. Further study indicated a strong linear function between the slope of the linear function and the elevation of the station, which lead us to correct the PM approach yielded result with the elevation of the study plot. Consequently, the modified PM approach with improved WGEN was applied to the estimation of ET for the area where observed meteorological data were absent or missing. In annual time scale the relative error between simulated observed ET was found less than 3% and 20% respectively in both monthly time scale and annual totals, which suggested the reliability of the modifications for both WGEN and PM ET0 module in hydrological process studies. But in daily time scale, further improvements are required.