Accurate estimation of crop water requirement is the basic step for designing and scheduling of irrigation and drainage projects. Evapotranspiration (ET) is a stochastic variable, therefore to reduce the risk level, it is needed to select and use proper evapotranspiration probability levels. In this research, 24 years meteorology data of Orumieh synoptic station have been used to estimate ET0. Also for investigation the effect of ET0 calculation methods on its value, FAO Penman-Monteith (FP-M) and Hargreaves-Samani (H-S) models were selected. Daily ET0 distribution curves with different probability levels for each method were extracted.Differences between maximum daily ET0 values (calculated by FP-M method) with 75% and 50% probability level was 1.0 mm/day (equivalent to 13%) and between 90% and 50% was 1.9 mm/day (24%). Similarly for H-S method the difference was 0.5 and 0.9 mm/day (8% and 15%), respectively. Also at a particular probability level, there was noticeable difference between values of FP-M and H-S methods.In order to understand the impacts of calculation period length on mean daily ET0 value, daily ET0 with 99% probability, and moving average of daily ET0 were calculated for 1, 3, 5, 7, 10, 15, 20, 25 and 30 day perrids. After determination of occurrence date of average maximum daily ET0 for the above periods, average of maximum daily ET0 with different probability levels for the periods of 1 to 30 days were determined. The occurrence of peak dates within 1 to 20 day periods, for FP-M method was 2 weeks after H-S method but for 25 and 30 day periods the dates were the same. The results showed that the peak mean daily ET0 of FP-M method for 5 day periods at 50% and 75% probability levels were 7.8 and 8.7 mm/day (12% difference) and for H-S method they were 6.1 and 6.6 mm/day (8% difference).