The Standardized Precipitation Index (SPI) is the most common index for drought monitoring. Although the calculation of this index is usually done by using the gamma distribution fitting of precipitation data, studies have shown that for accurate monitoring of drought, the optimal distribution of precipitation in each month should be determined. On the other hand, in non-stationary time series, it is not possible to determine the proper parameters of probability distributions. Hence, in recent years, the non-parametric SPI has been introduced, in which the parametric distribution is replaced by an empirical probability distribution. However, in national studies, there is still insufficient attention to non-parametric drought indicators. Therefore, in this paper, the performance of the nonparametric SPI was evaluated for drought monitoring over Gorganroud basin during 1972 to 2015 and the results were compared with the parametric SPI. The SDAT software package was also introduced to calculate nonparametric drought indices. The results of this study showed that time series of the parametric approach based on the best probability distributions and the nonparametric approach are well correlated with each other. The correlation coefficient between the two-time series is equal to one. Drought events have been detected at the same time based on the two approaches. Based on the SPI12 in both parametric and non-parametric approaches, the longest drought occurred in the basin between February 1983 and March 1984 for a period of 14 months, during this period, the basin suffered severe drought.