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Iran is located in arid and semi-arid regions in the world and therefore, drought monitoring has an important role in there. Drought assessment is conventionally based on drought indices such as Reconnaissance Drought Index (RDI). On the basis of definition, Reconnaissance Drought Index is estimated by fitting Log-normal distribution on the ratio of precipitation to potential evapotranspiration (calculated by Thornthwaite method). In this research, the effect of the change of distribution function and potential evapotranspiration calculation method was investigated in estimating Reconnaissance Drought Index in Iran. For this purpose, meteorological data of 30 synoptic stations were used in the period of 1960-2014. The potential evapotranspiration was calculated using the FAO Penman-Monteith as standard method. Beside standard method, 20 different methods were used to calculate evapotranspiration value. The superior evapotranspiration calculating method was obtained using Root Mean Square Error statistic. Then, the Reconnaissance Drought Index values were calculated using different evapotranspiration values and by fitting several probability distribution functions. On the basis of obtained results, the change of probability distribution function and evapotranspiration calculation method can affect the determining Reconnaissance Drought Index. Also, the comparison results of the calculated Reconnaissance Drought Index based on the fitting the superior probability distribution function on the ratio of precipitation to potential evapotranspiration which was calculated using superior and standard methods showed that the FAO Penman-Monteith method should be used in 90% of the studied stations to calculate Reconnaissance Drought Index in cases of data available.