The critical heat flux (CHF) of subcooled boiling is theoretically predicted by means of the microlayer model. The enhancement of heat transfer for subcooled boiling is mainly due to the augmented heat convection caused by the forming and collapsing of individual bubbles. For a uniform heat flux surface, CHF approaches a constant in the high subcooling region. For a uniform temperature surface, CHF increases with the subcooling. The evaporative heat transfer becomes small and the total heat flux is mainly due to the heat conduction outside the evaporating area as the subcooling is increased. The boiling crisis(CHF) is caused by the local dryout of microlayer at low subcooling and by the rapid increase of the duration of bubble condensation with the increase of wall superheat at high subcooling.