Laminar natural convection of air along a vertical square duct open at both ends has been investigated for constant temperature and constant heat flux boundary conditions on the duct walls. The velocity of entering air at the bottom of the duct is assumed uniform at atmospheric pressure or the acceleration head of the entering fluid. The three-dimensional differential equations for momentum and energy have been solved numerically by a strongly implicit finite-difference procedure. The axial development of flow and heat transfer rates are presented, A curve fit for the mean heat transfer coefficient in the Rayleigh number range of 10 to 1000 results in the following relations: