[en] Experiments on Critical Heat Flux (CHF) of Freon-12 have been carried out in the rectangular vertical channels with gap sizes of 1 mm, 2 mm, and 3 mm, respectively. Analysis of CHF data in the different channels shows that CHF decreases as the increase of pressure, and while the exit quality rises, CHF falls down quickly. Meanwhile, CHF goes up as the mass velocity increases when the exit quality is lower, and contrarily, when the exit quality is higher, the increase of mass velocity results in the decrease of CHF. According to the analysis results, a conclusion can be drawn that gap sizes almost have no effect on the CHF in rectangular channels with gaps from 1 mm to 3 mm under the same condition in the experiment. (authors)