Abstract: The heat transfer of natural convection between air and micro-wires with diameters ranging from 39.9 μm to 350.1 μm, placed at horizontally, is investigated.The flow is considered to be two dimensional and incompressible, and the Boussinesq approximation is employed to deal with the bouncy generated flow.The SIMPLER algorithm is used for the pressure-velocity coupling, and the influences of bouncy, the thickness of boundary layer and the edge effect on the natural convection heat transfer are considered during the simulation.The Joule heat method is employed to obtain the Nusselt numbers in the experiment.The results are compared with the predictions of Maigan's classical correlations and the experimental results, and the numerical results are in accordance with the experimental data.The investigations show that the heat transfer coefficient of natural convection increases with the decrease of the diameter of the micro-wire.These phenomena may be due to the enhancement of the natural heat transfer caused by the thinner boundary layer which resulted from the edge effect when the diameter of the micro-wire becomes smaller.However, the Nu number of natural convectionincreases at first and then decreases, and the deviation of the numerical results from the classical correlation of Maigan becomes lager with the decrease of the diameter of the micro-wire.