Abstract: Based on the daily meteorological data in Aksu water balance experimental station in Tarim River Basin from 1989 to 1996, the reference evapotranspirations (ET₀) calculated by the Penman-Monteith and Penman methods were compared. The results showed that the annual ET₀ calculated by Penman method was a little higher than that obtained by Penman-Monteith method. The absolute deviation of yearly ET₀ between the two methods was 42~128 mm, the yearly relative deviation was 3.3%~9.8%. The difference of monthly ET₀ was smaller. The absolute deviation of monthly ET₀ between two methods was above zero in January, February and December, and was below zero from March to November. The relative deviation of monthly ET₀ was bigger in January and December, was smaller in February and November, the variation of the relative deviation of monthly ET₀ was small in other months. The main cause of the deviation of ET₀ was that different equations and parameters were used to calculate the radiation item and aerodynamic term in the equations. The correlation between ET₀ calculated by two methods had linear relationship.