Sensitivity analysis of reference crop evapotranspiration (ET0) in Iran is an important necessity due to better management of water in such an arid and semi-arid country. It is important to analyze the sensitivity of ET0 to weather parameters as climate seems to be changed to some degree everywhere. In this study, sensitivity of ET0 to climatic variables at the eight selected stations of Iran was investigated. To estimate the ET0, the most recommended form of the FAO-56 Penman-Monteith method was used. Sensitivity of ET0 was analyzed in terms of change in mean air temperature (T-mean), actual vapor pressure (e(a)), wind speed (u(2)) and net solar radiation (R-n) within a range of +/- 20 %, in 5 % steps, from their corresponding normal long-term values (1965-2005). Results showed that ET0 was most sensitive to T-mean at the six of the stations in annual time scale. Increase in T-mean by +20 %, caused the most increase in ET0 at Chabahar. In contrast ET0 was least sensitive to e(a) in most selected stations. In this study, we investigated the interaction parameters in different scenarios on ET0 at all stations in monthly and annual time scale. Results showed in annual time scale increasing T-mean and WS by 20 % and decreasing e(a) by 20 % yielded ET0 increasing 36.4 % at Chabahar. But, in monthly time scale 10 % increase in T-mean and WS as well as 10 % decrease in R-n yielded ET0 to be increase to about 30.5 % in December at Bandar Anzali.