Abstract: The critical heat flux（CHF） under natural circulation condition in rectangular channel was investigated in this paper.It was found that the dryup of a liquid layer in annular flow caused the insufficient cooling of heating wall and the rise of wall surface temperature.Then the boiling crisis occurred at the outlet of test section.Accord to the trend of the wall temperature rise and steam quality,the category of boiling crisis is considered to be dry-out.In addition when the boiling crisis occurs,the natural circulation flowrate was found to be increased.Based on the theoretical analysis,the sharp decreasing of the friction drop results in the rise of natural circulation flowrate when annular flow turns to dispersed flow.