In this paper, the conjugate conduction-free convection heat transfer from a horizontal cylinder is numerically and experimentally investigated. In the theoretical study, the unsteady two-dimensional conduction equation for the cylindrical wall and the unsteady laminar two-dimensional governing equations for the flowing fluid are solved simultaneously using the finite-difference method. The effects of controlling parameters, such as Rayleigh number, thermal conductivity ratio and diameter ratio on heat transfer process have been investigated. The study has shown that the heat transfer rate increases with increasing Rayleigh number and thermal conductivity ratio. The study has also discussed the existence of critical radius for some Rayleigh numbers and thermal conductivity ratios. The streamlines and isotherms are plotted for some cases to show the details of the velocity and thermal fields. For verification of the numerical model, the present numerical results are compared with previously published experimental and theoretical data where good agreements were found. In the experimental study, the heat transfer from a hollow horizontal aluminum cylinder with diameter ratio of 2 and heated from its inner surface at constant temperature is investigated. The obtained experimental results are used to further assess the validity of the numerical results and acceptable agreement has been found.