Abstract: Based on the lump parameter models developed in this paper, general transfer functions for transient responses of fluid outlet temperatures of two - stream plate - fin heat exchanger to changes of either cold fluid inlet temperature or cold fluid flow are derived. The models have been verified by experiments. Detailed analyses show that applicable linearized ranges for transfer functions are unlimited for fluid temperature changes, or △m/m < 30% for fluid flow changes. When both fluid temperature and mass flow change simultaneously, the applicable linearized ranges are limited to their 20 percent changes of these two parameters.