Abstract: A new model is proposed for the prediction of saturated flow boiling heat transfer inside vertical tubes. The model assumes the forced convection and nucleate boiling coexist in the annular flow regime. Conservation of mass, momentum, and energy are used to solve for the liquid film thickness, velocity, and temperature. The heat transfer due to nucleate boiling is determined from the frequency and diameter of bubble departure, the density of active nucleation sites, and suppression factor. The overall heat transfer coefficients are calculated from the total heat flux partitioned into the two components of forced convection and nucleate boiling. Predictions of the model agree with the experimental data within the error of ±20%. The numerical results indicate that the nucleate boiling contribution to the overall heat transfer coefficients decreases from 20% to 1% with quality increasing.