In this paper we propose a microlayer model in which the partial dryout of the liquid microlayer underneath the individual bubbles is regarded as the mechanism of critical heat flux (CHF). By considering the evaporation of the liquid microlayer and the dynamics of bubbles, both the initial growth and the departure process of bubbles are analyzed, then the dryout area and the heat flux are formulated as functions of superheat ΔTs and the density of active sites N. Using the experimental data of N available in the literature, the CHF has been derived as the maximum value of the heat flux. The prediction of this model shows good agreement with some previous experimental data.