Abstract: Heat exchanger design is one of the most important parts of process design and integration. The value of heat transfer temperature difference ΔT greatly affect the trade-off between investment and energy-saving, which deponds upon the ratio of energy price / equipment price. This paper studied and developed the thermal exergy pricing method and heat exchanger costing method according to exergo-economic theory. Based on these, futher developed the mathematical model of optimal temperature difference ΔTopt of heat transfer, as well as the cerresponding computer aided optimal design software. The application shows that the design which takes ΔTopt can result in much higher energy efficiency and economic benefits.