An experimental study has been made of natural convection heat transfer from the fin-flat, one of the component surfaces of a fin array ; and the effect of fin geometry and temperature on the heat transfer coefficient has been clarified. From these data and the results of the author's previous reports, an empirical formula for the average Nusselt number of vertical open-channel is derived. Furthermore, a comparison in heat transfer between open-channels and rectangular ducts has been carried out. The existence of the minimum heat transfer coefficient for the fin-length is qualitatively confirmed.