The objective of this paper is to propose a method for synthesizing the optimal mass exchange network (MEN). In order to synthesize optimal MEN, it is necessary to determine the network structure and its operating conditions. The material and operating unit nodes that are essential to use the Process graph theory (Friedler et al., 1992) are defined. The maximal structure of MEN synthesis problem can be easily obtained using the P-graph theory. Furthermore, only feasible structures can be extracted from the maximal structure, the size of the search space is extremely reduced. The feasible structures are formulated as nonlinear programming problems (NLP) and their operating conditions are determined by optimization of NLP.