Numerical investigation of capillary annular condensation is conducted. The conservation equations have been solved in both phases for variable heat flux imposed in outer wall of the micro-condenser. The results indicate that the film thickness, the Nusselt number and the condensation length are determined by the physical and geometrical parameters. A larger exponent of heat flux imposed on the outer wall can result in a longer film thickness of the condensate. In addition, the simulation results of thermal steady of annular condensation flow in rectangular microchannels are compared and evaluated with the different studies devoted to the calculation of condensation heat transfer in microchannels.