A new mechanistic critical heat flux (CHF) model has been developed for flow boiling CHF data of low-pressure (i.e., 0.2 to 4.0 MPa), low-mass-flux (i.e., 189 to 789 kg/m2s), and high-quality conditions. In general, CHF at these conditions associates with the flow regime of annular flow. This model assumes that the Helmholtz instability at the liquid-vapor interface of annular flow triggers the onset of CHF. CHF is the energy required to dryout the liquid film isolated by flow instability. With five empirical constants to properly correlate the liquid-vapor configurations of annular flow in the steam-water systems, the model successfully achieves a mean deviation error of 10.2% over a CHF data set consisting of 733 CHF data. The prediction of this model is more accurate than those of Biasi and Bowring correlations at the aforementioned low-pressure and low-mass-flux conditions.