Abstract: This research studied the flow boiling heat transfer characteristics in vertical rectangular channel with 2mm gap width under the thermosiphon circulation.It was found that convective evaporation heat transfer somehtimes was in a transitional flow state,the calculation method was not reported.An equation for convective evaporation was developed and was suitable for the calculations of laminar,transition and turbulent flow states.The absolute deviation of the calculated flow boiling heat transfer coefficient from the experimental data was 14.9% by this method.