Natural convection heat transfer from a finned horizontal cylinder has been studied numerically. The present numerical investigation is able to capture a complete picture of natural convection over an annular finned cylinder from where easy visualization about the plume structure and flow field over the tube-fin surface can be obtained pictorially. In the present study, numerical simulations of full Navier-Stokes equation along with the energy equation have been conducted for a horizontal tube with annular fins of constant thickness for the laminar range of 5 <= Ra-s <= 10(8). Optimization studies on the conjugate heat transfer characteristics have been conducted to find the best fin spacing and fin-to-tube diameter ratio for maximum heat transfer. With addition of fins to the heated surface, heat transfer increases and gets a maximum value, then heat transfer starts to decrease. The optimum fin spacing for maximum heat transfer varies between 5 mm and 6 mm roughly for Raleigh number range of 5 <= Ra-s <= 10(8). The effect of the parameters like Did, S/d and Ra-s on Nus and Nu(D) are analyzed and a correlation for average Nusselt number has been developed for natural convection from a finned horizontal cylinder. A correlation for optimum fin spacing has also been developed as a function of Ra-s and Did which may be useful for design of various heat exchangers. (C) 2016 Elsevier Ltd. All rights reserved.