Abstract: In order to reveal the difference between hourly reference crop evapotranspiration(ET0) calculated by FAO56 Penman- Monteith equation and that by ASCE Penman-Monteith , as well as the 24-hour accumulated daily ET0, well watered lysimeter experiment were carried out on turf grass. Hourly ET0 and 24-hour accumulated daily ET0 were calculated based on hourly meteorological data and evaluated by using daily observed grass evapotranspiration derived from well watered lysimeter experiment. There was a certain difference between ET0 calculated by FAO56 Penman-Monteith equation and that by ASCE Penman-Monteith equation, and the difference was more significant when the ET0 values were relative higher. Hourly ET0 were tested to be lower for FAO56 Penman-Monteith equation than for ASCE Penman-Monteith equation in daytime, but just reversed at night. That was ascribed to the difference in Cd values. It was a pity to say that the equation mended by ASCE could not result in improvement in hourly ET0 calculation, when the 24-hour accumulated daily ET0 were compared with observed daily grass evapotranspiration. Daily ET0 calculated by Penman-Monteith equation based on meteorological data at daily time step were test to be the most consistent with that by lysimeter observed. Further evaluation of either FAO56 Penman-Monteith equation or ASCE Penman-Monteith equation was urgent in China, based on hourly or long series daily observed grass evapotranspiration.