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A three-dimensional model in rectangular microchannels with constant heat flux is developed to predict steady annular condensation. The condensate flow field on the side wall, which is dominated by surface tension, is divided into two regions: the thin–film region and the meniscus region. The momentum and mass equations, in both the vapor and meniscus regions, along with the film thickness equation in thin–film region are solved numerically. The distribution of the meniscus curvature radius, thickness of the condensate film, heat transfer coefficient, and wall temperature are all determined. The results indicate that with the development of condensation, the condensate in the thin–film assumes a convex profile shape at the side wall, with the crest located at the midpoint of the side wall. The film thickness in the thin-film region increases at upstream locations and decreases as the flow moves downstream. The average heat transfer coefficient in the thin-film region is much larger than that occurring in ...