The local heat transfer coefficient of the free convection of air around a horizontal thin wire of 0.470 mm in diameter is measured by means of a Mach-Zehnder interferometer. Then the full Navier-Stokes and energy equations are numerically solved in a field including the buoyant plume for Ra_d = 0.37, 1.0 and Pr = 0.7 by means of 'donor cell' method. Both results for the local heat transfer coefficient agree well mutually. A new correlation equation for the average heat transfer coefficient is proposed by referring available theoretical and experimental data in the range 10⁻⁸ < Ra_d < 10⁶.