Daily reference crop evapotranspiration ET ο was calculated by means of the Penman-Monteith method, and the monthly ET ο value was accumulated based on two different time steps of daily and monthly mean meteorological data. The adopted data include meteorological records from Yanzhou (1981-1986)and Heze weather station (1981-1986), Dingtao weather station (1997-2002). The differences of ET ο value calculated with two different scales of meteorological data were evaluated. The results show that: First, monthly ET ο values calculated with two time steps weather data have a high correlation, and the correlation coefficient between them is about 0.97. Second, monthly ET ο value calculated with mean monthly meteorological data is 3% lower than ET ο calculated by the daily mean meteorological data. Consequently the paper comes to the conclusion that ET ο can be calculated with mean monthly meteorological data instead of daily mean meteorological data by means of the Penman-Monteith equation.