In this paper, a numerical investigation of air side performance of strip fin surface is presented. Three-dimensional numerical computation was made for a model of a two-row finned tube heat exchanger. The tube configuration is simulated with step-wise approximation, and the fin efficiency is also calculated with conjugated computation. It is showed that the strip fin with strips located in the rear part of the fin surface has a better performance than that with strips in the front part. Detailed discussion is provided to reveal the basic mechanism of such phenomenon. The present numerical work once again shows the correctness of the filed, synergy principle, and useful information is provided on where should the enhancement element be positioned.