To understand the mechanism of subcooled CHF(Critical Heat Flux), the experiment of pool boiling of water was conducted on a cooper heated surface at subcooled conditions under atmospheric pressure. CHF increases with subcooling and agree well with available empirical correlations of Zuber et al., Kutateladze, and Ivey and Morris. At low subcooling less than 55 K, a vapor mass was formed to cover the heated surface and the critical condition is understood basing on the macrolayer consumption model of Katto and his coworkers by employing newly defined time period for macrolayer consumption and by assuming that the initial dry area fraction of the heated surface depends on CHF. At high subcooling more than 55K, it was found that a vapor mass hardly cover the heated surface, so the macrolayer model is difficult to apply from its physical basis. The mechanism of CHF remains unknown in the present study.