Paper mills use and discharge lots of water. And so now the papermaking industry could be classified into major water consuming industry In order to analyze the process water network and to establish the mass, water balance of duplex board mill, computer aided simulation was made using water pinch method. Based on the pinch analysis results, reuse of process water, after regenerating by microfilter as much as $140;m^3/hr$, could be suggested without significant accumulation of contaminants in process water. According to this suggestion about $3000;m^3/day$ of recycled process water could be sub stituted by regenerated water and consequently $30\%$ of energy cost is expected to be reduced.