Two-dimensional laminar natural-convection heat transfer in air around horizontal ducts with rectangular and square cross sections is studied numerically. Different aspect ratios are used for wide ranges of Rayleigh numbers. Results are presented in the form of streamlines and isothermal plots around the circumference of the ducts. The computational procedure is based on the finite-element technique. Temperature and velocity profiles are obtained near each surface of the ducts. Reverse flow and circulations are observed at high aspect ratios. Heat transfer data are generated and presented in terms of Nusselt number versus Rayleigh number for different aspect ratios. Correlation covering the aspect ratios used is obtained in dimensionless form of Nusselt number, Rayleigh number, and aspect ratio.