Based on the principle of heat transfer and the error estimation theory, the present paper, by means of optimization, introduces a method to treat heat-transfer test data without measuring the wall temperature. If the local heat-transfer coefficients on both sides of the heat exchanger have certain demensionless forms and the temperature of the inlet and outlet of both fluids and the flow rates of both fluids are measured, no matter how great or small the ratio of their coefficients may be, their coefficients can be separated from the overall heat-transfer coefficient and their corresponding demensionless forms may be obtained. Furthermore, the errors of their coefficients can be estimated from the error of the overall heat-transfer coefficient.