The vertical three-dimensional finned tube is one kind of strengthened tubes that effectively enhance heat transfer. In this paper, the characteristics of heat transfer on the exterior enhanced surface of a vertical 3-D finned tube were studied under natural convection conditions. The effects of varied fin height, fin width, axial fin pitch and circular fin pitch on Nusselt (Nu) number were presented. The results showed that the natural convective heat transfer performance of vertical 3-D finned tube was much better than that of a smooth tube. Under the study range, Nu number of the vertical 3-D finned tube was 2.83-2.97 times greater than that of the smooth tube. With the increase of fin height and the decrease of axial fin pitch, Nu number is increased, but the change of fin width and circular fin pitch has a little influence on the Nu number. Finally, the empirical correlation of air side free convection heat transfer was proposed for vertical 3-D finned tube, the predicted values of Nu number in agreement with the experimental values within the error of ± 11.3 %.