In this paper, a thermodynamic analysis of non-isothermal mixing's influence on the energy target of water-using network is presented. Firstly, water streams in the network are divided into two categories, and then based on the classification all the non-isothermal mixing patterns between two streams are defined. Through thermodynamic analysis from energy composite curve of hot and cold water streams, the influences of non-isothermal mixing on the energy target of water-using network are explored and some mixing rules are obtained, which can be used to simplify the heat exchanger network of a given water-using system and improve the system's energy performance through identifying the beneficial non-isothermal mixings. The applicability of these rules is illustrated by an example in the paper and the result is very encouraging. (C) 2013 Elsevier Ltd. All rights reserved.