The critical heat flux (CHF) of subcooled boiling is theoretically predicted by using the microlayer model. The enhancement of heat transfer for subcooled boiling is mainly contributed by the augmented heat convection caused by the forming and collapsing of individual bubbles. For uniform heat flux surface, the CHF approaches a constant at high subcooling region. For uniform temperature surface, the CHF increases with the subcooling. The evaporative heat transfer becomes small and the total heat flux is mainly contributed by 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.