Determination of Nusselt and Rayleigh numbers during natural convection heat transfer from horizontal cylinders are investigated experimentally and numerically. Experiments are done by means of the test cabin inside a conditioned room at different environmental and surface temperatures. The environmental and cylinder surface temperatures are ranging between 10-40 °C and 20-60 °C respectively. 1 m long horizontal copper cylinder is used and has outer diameters of 4.8 mm including centered silicone covered cylindrical resistant wires inside it. The experimental apparatus is designed to capable of changing the different operating parameters such as heat flux and environmental temperature. The existence of the gap on the closed surfaces in the test cabin which can cause a stack effect, affected temperature and velocity fields, disturbance of the natural convection condition is also checked. The detailed description of design and development of the test apparatus, control devices, instrumentation, and the experimental procedure are reported and the study of experimental setups from the available literature survey with the existing one are compared in this paper. The uncertainty analysis method proposed by Kline and McClintock is used and explained elaborately. Detailed information and algorithm of numerical method are given to ease the understanding of the numerical part of study. Alteration of Nusselt numbers with Rayleigh numbers, the temperature distribution on the heated horizontal cylinder surface by means of Fluent CFD program are shown in the paper. In addition to this, Morgan's correlation is used for the comparison of Nusselt number and is found in good agreement with the experimental results. Copyright © 2010 by ASME.