Natural convection heat transfer in annular fin-arrays mounted on a horizontal cylinder was experimentally investigated. An experimental set-up was constructed to investigate heat transfer characteristics of 18 sets of annular fin-arrays mounted on a horizontal cylinder of 24.9-mm diameter in atmospheric conditions. Keeping the fin thickness fixed at 1 mm, fin diameter is varied from 35 mm to 125 mm and fin spacing is varied from 3.6 mm to 31.7 mm. The base-to-ambient temperature difference was also varied with a calibrated wattmeter ranging from 25 W to 150 W. The results have shown that the convection heat transfer rate from the fin arrays depends on fin diameter, fin spacing and base-to-ambient temperature difference. In addition, for every fin diameter, for a given base-to-ambient temperature difference, there exists an optimum value for the fin spacing for which the heat transfer rate from the fin array is maximised. Experimental results show that, for practical engineering applications, the optimum fin spacing may be taken approximately as 8 mm. A scale analysis is also performed in order to estimate order-of-magnitude of optimum fin spacing at a given fin diameter and base-to-ambient temperature difference. The correlation obtained from scale analysis is the result of limited number of experiments. This correlation may be generalised by applying the order-of-magnitude analysis developed in this work for wider range of experiments.