Abstarct: One of the most important parameters in hydrocarbon reservoir evaluation is water saturation, which commonly determines from electrical logs data and respective equations, but these methods conclude assumptions that are caused uncertenity in the results. Whereas core data have high accuracy, the main target of this study is calculation of water saturation in different depths of reservoir, by using core data and improved method. In this study, estimation of water saturation has been done by combination of capillary pressure data obtained by mercury injection and classification of carbonate rocks using hydraulic flow units, and relation between leverett function and normalized water saturation. Finally the calculated water saturation is compared with water saturation from log data. The results show that the presented method can be used for determination of water saturation in reservoir. Also, the results show acceptable correlation between irreducible water saturation and reservoir quality index in differens hydraulic flow units, these relationships can be used for estimation of irreducible water saturation from porosity and permeability data in each part of well.