Visualization experiments were conducted to study the condensation flow in rectangular silicon microchannels with hydraulic diameter of 90.6 μm and width/depth ratio of 9.668.In such wide rectangular microchannels，droplet-annular compound flow，injection flow and slug-bubbly flow were observed.In the droplet-annular compound flow regime，the vertical wall of the channel was completely covered by the condensate film.But on the long sidewall of the channel，droplet condensation still existed.The occurrence of injection flow would be postponed with increasing inlet vapor Reynolds number.And it was confirmed that the cross-section shape of the microchannel had a significant effect on the instability of condensation flow.Slug-bubbly flow occurred after injection flow.Along the condensation stream，the slug would contract into round bubble due to surface tension.The average heat transfer coefficient of the channel increased with increasing inlet vapor Reynolds number.