Aim of this study is to estimate the optimized volume of reservoir in un-gaged basins and extract the estimation pattern of reservoir capacity for other regions. For this purpose, Ziarat was divided to 9 sub-basins using ArcGIS and the physiographical parameters was determined for each of them. for modelling of volume variation, demand before and after rain and also product before and after spill, were considered as different of Possible scenarios. Time history for amount of volume, was obtained as an equation according to other variables. The optimized volume calculated by mentioned equation as objective function and constrains.i.e. cost, demand and spill for each sub-basin. These volumes vary from 700 to 5000Mm3 and significant relation between volume and curve number index was observed. To extract a relation for prediction of reservoir optimized volume in un-gaged regions, regional parameters and optimized volume was considered and 6 equations was prepared by dimensional analysis and different functions. in this equations, 12 rainfall of 24-h probable maximum event were used for 4 consecutive years (2012-2015). Finally, using R-square and standard deviation error, the more optimized equation was presented.