An analysis for the effect of wall-surface curvature on gas microflow is performed to study the natural convection in an open-ended vertical annular microchannel with an isothermally heated inside wall. The fully developed solutions of the velocity, temperature, flow rate, shear stress, and heat flux are derived analytically and presented for air and various Surfaces at the standard reference state. Results show that wall-surface Curvature has a significant effect. This results in a nonlinear behavior in the temperature, which seems difficult to appear in a parallel-plate microchannel. Under certain rarefaction and fluid-wall interaction conditions, by decreasing the value of the curvature radius ratio, it is possible to obtain both reduced flow drag and enhanced heat transfer. Crown Copyright (C) 2008 Published by Elsevier Ltd. All rights reserved.