One of the most important parameters in hydrocarbon reservoir evaluation is irreducible water saturation; commonly determined from capillary pressure or relative permeability experiments. Real reservoir rock samples are needed for doing these experiments. Usually such samples are hardly available and their preparation is expensive. In this study, estimation of irreducible water saturation has been done by combining capillary pressure data and hydraulic flow units. In this article the number of hydraulic flow units has been determined based on measured porosity and permeability on the samples of a carbonate reservoir.First by calculating reservoir quality index and pore to matrix ratio, flow zone indicator determined for each sample and then by histogram and sum square errors analyses the number of hydraulic flow units was clarified.