The optimization of heat exchanger networks (HEN) is a typical MINLP problem. For large-scale HEN, it is difficult to solve this problem globally. After optimization, the large-scale HEN is divided into several independent sub-networks automatically. The sub-network is defined as a part of the HEN in which the streams have no heat transfer with the streams outside the sub-network. If a HEN can be divided into two or more sub-networks, then, these sub-networks are independent from each other. Based on optimization of sub-networks, a new method which can solve large-scale HEN problem efficiently is proposed.