In general, offset strip fin (OSF) used in plate-fin heat exchangers is able to provide a greater heat transfer coefficient than the plain fin with the same cross section, but it will also cause the increase of flow friction and pressure drop owing to the fin offset. A new parameter denoted by Ψ*, called relative entropy generation distribution factor, is proposed in this paper to comprehensively reflect the thermodynamic performance of different passage structures in plate-fin heat exchanger. This parameter physically represents relative changes of entropy generation and irreversibility, which are induced by both heat transfer and friction loss due to the utilization of OSF fins. The high magnitude of Ψ* represents a beneficial contribution of OSF with higher degree of the heat transfer enhancement. The proposed method is more reasonable and comprehensive than either the conventional augmentation entropy generation number, Ns,a, or the entropy generation distribution factor, ψ, to evaluate the heat transfer enhancement for OSF cores subject to various operating conditions. With the proposed method, the relative effects of the geometrical parameters of OSF fins, such as the fin thickness-to-height ratio α, fin density γ, and fin thickness-to-length ratio δ, on the heat transfer enhancement are discussed in detail. The results show that relatively small δ results in a better performance, while the parameter α or γ, which contribute to a higher degree of heat transfer enhancement of OSF fin, should be determined after the selection of the other two geometric parameters.