As an important consideration in the design of plate-fin heat exchangers, the selection of plate-fin surfaces is associated with the estimation of the fin performance in many cases. The fin performance of offset strip fin (OSF) and plain fin is numerically investigated with well-validated 3D models in the present study. The comparative analysis shows that the conventional fin efficiency and fin effectiveness concepts provide an incomplete assessment of the fin performance of the fins, and lead to impractical suggestions of using OSF fin. Further investigation indicates that the idealization of uniform heat transfer coefficient over all the surfaces in fin channel, which runs through the conventional concepts, is untenable, and strongly restricts the fin performance analysis. An actual fin effectiveness is then proposed to measure the fin performance. It physically represents the ratio of the heat flux over the fin surfaces and that over the primary surfaces in the fin channel. With this method, the effects of the geometrical parameters of the OSF are discussed carefully. The results show that there exists a specific fin thickness-to-height ratio α and fin density γ, which contribute to the highest fin performance for a given mass flux, and the optimal γ (or α) increases (or decreases) as mass flux increases. The OSF fins with relatively large fin thickness-to-length ratio δ perform better in low Re region and the optimum δ decreases with the increasing Re number.