An experimental study was conducted to investigate the effects of a variety of hydrodynamic parameters on the boiling critical heat flux (CHF) of mist cooling for a large-size hot surface in the low and moderate mass flux regions. The tested mass flux ranged from 0.015 kg/m(2) s to 4.0 kg/m(2) s, and the mean droplet diameter from 12 to 198 mum. The experimental results indicate that the mechanism of the critical heat flux for mist cooling can be divided into two different patterns according to the mass flux of the mist flow. One is the liquid droplets drying pattern that occurs when the droplets that fall on the hot surface are completely evaporated in the low mass flux range. Another is the liquid film fragmentation pattern that occurs when the liquid film fragments at the edge of the hot surface in the moderate mass flux range. For the liquid droplets drying pattern, the critical heat flux depends on both the mass flux and the mean droplet diameter, and it increases linearly with increasing mass flux. However, for the liquid fragmentation pattern, the critical heat flux depends basically on the mass flux, and it increases weakly with increasing mass flux.