A new dynamic microlayer model has been proposed to predict theoretically the heat flux in fully developed nucleate boiling regions including critical heat flux (CHF). In this model, the heat transfer with boiling is mainly attributed to the evaporation of the microlayers which are periodically formed while the individual bubbles are forming. Since the initial microlayer thickness becomes thinner with the increase of wall superheat, both the local evaporation and the partial dryout speed of the microlayer increase. As a result, the time-averaged heat flux during the period of individual bubble has a maximum point, the CHF, at the predicted continuous boiling curve.