Abstract: In order to research the heat transfer characteristics of annular flow in the rectangular narrow channel, a mathematical model of annular flow in rectangular narrow channel is developed based on the mass, momentum and energy conversation equations of liquid film, as well the momentum conversation equation of vapor core. Through numerically solving the closure equations, the boiling heat transfer coefficients of annular flow in the rectangular narrow channel is obtained, and the effects of heat flux, mass flow rate and rectangular channel size on the liquid film thickness are analyzed. The results show that the present model can well predict the two-phase heat transfer coefficient, and the deviation is within ±30%. The heat flux and rectangular channel size have a big effect on the liquid film thickness.