Theoretical analysis and experimental confirmation for the principle to improve the thermal performance of heat exchangers is performed in this paper, The more uniform the temperature difference field (TDF), the higher the effectiveness of heat exchanger for the fixed N-tu and C-r. The uniformity of the TDF and the effectiveness of 13 types of heat exchangers are studied analytically and numerically, and the results support the uniformity principle of TDF. Further verification is given by the asymptotic solution for TDF in terms of a recurrence formula of heat transfer area distribution for the same kind of heat exchanger. The analysis of entropy generation caused by the heat transfer indicates that the uniformity principle of TDF satisfies the second law of thermodynamics. The results of the experimental setup, governed by the uniformity principle of TDF, show that the effectiveness increases with the increase of the uniformity of TDF. Heat exchanger effectiveness for the best flow distribution was found to be 11.2% greater than that of the conventional flow distribution without associated increase in pressure drop. Two ways, redistributing heat transfer areas and varying the connection between tubes, are presented for improving the uniformity of TDF and increasing effectiveness for the crossflow heat exchangers.