Gravity separator (dry pond) is one of the common separation equipment that is widely used in pistachio processing units. In this study, the performance evaluation of the gravity separator in separating of blank pistachio nuts as well as other low density part of crop was aimed. For this purpose, complete separation of the pistachio nuts by water was considered as separation criteria, and based on this, the relative separation index (R.S.I.) was defined to evaluate the performance of the gravity separator. Gravity separator performance in separation of two varieties pistachio nuts (Fandoghi and Momtaz) was evaluated using the index for different outlet gates adjustments including regular adjustment (both outlet gates in second position). This study indicated that Relative Separation Index of Momtaz variety was significantly higher than Fandoghi. Regardless of the variety, the highest value of R.S.I. was 64.5%. The results showed increasing of R.S.I. from 79% to 93% (compared with water separation) can be achieved by changing the facility adjustment. On the other hand, it was revealed that by changing the settings of the dry pond, although better performance can be achieved in the Relative Separation Index, but this leads to a sharp increase in the quantity of rejected output from the apparatus.