A three dimensional model in rectangular microchannels with constant heat flux was developed to predict the steady annular condensation.The momentum and mass conservation equations both in the vapor and meniscus regions and the film thickness equation in the thin film region were numerically solved.The distribution of the meniscus curvature radius，thickness of the thin liquid film，heat transfer coefficient and wall temperature were obtained.The film thickness in the thin film region increased in the head stream to a maximum，and then decreased in the downstream.The average heat transfer coefficient in the thin film region was much larger than that in the meniscus.The highest heat transfer coefficient in a cross section appeared at the joint of the thin film region and meniscus，where wall temperature was the highest.The circumferential average heat transfer coefficient decreased drastically at the head stream to a lowest value.After that，the circumferential average heat transfer coefficient remained almost constant while approaching the end of the annular flow，then it decreased again.