A numerical study of the flow structure upstream a single plate-finned tube is presented in this paper. Investigations have been carried out for three different fin-spacings E/D= 0.13, 0.2, 0.27 and for different Reynolds numbers by means of a computational fluid dynamic code. The calculations are compared with flow visualization performed by PIV (Particle Image Velocimetry) system. It is shown that calculations reproduce correctly the flow topology and results are very close to the experimental data. A strong dependence of the flow structure on the fin-spacing and on the Reynolds number is observed as experimental results pointed it out.