A mechanistic model, which is widely applicable to the flow boiling regime, is developed to predict the critical heat flux (CHF) in the In-Vessel Retention (IVR) configuration. The model is based on five general equations describing the CHF mechanism and four primary CHF variables: vapor velocity, liquid velocity, microlayer thickness and the slug length. The CHF mechanism of liquid film dryout underneath the slug is considered. Velocities of vapor and liquid are determined by the Karman velocity distribution and the force balance between buoyancy and drag force. The microlayer thickness is defined by Cheung and Haddad (Cheung and Haddad, 1997) model, based on the Helmholtz instability for the vapor stem located in the microlayer. The slug length is postulated to be the critical Helmholtz wavelength. The solution is numerically obtained starting from seven scattered input parameters: mass flux, local quality, pressure, inclination angle, gap size, working fluid and heater material. In the model, four unknown constants are used for the premature CHF due to heater deformation, the minimum length of slug and the approximation of microlayer thickness and liquid velocity. To find the best-fitted values of the unknown constants, the URANIE code, developed by Commissariat a l'Energie Atomique (CEA), is used. The CHF predicted by the new model is compared with the integrated IVR-CHF database, including experimental data from KAIST, CEA (SULTAN experiments), UCSB (ULPU experiments) and MIT. Starting from 278 experimental data points, the CHF model is affected by a root-mean-square (RMS) error of 14.8%. The CHF predicted by the model is in good agreement with the experimental IVR-CHF database, except for the condition of high mass flux conditions (> 500 kg/m(2) s) and low inclination angle (< 16 degrees). Further improvement of the model is suggested to cover this range and reduce the RMS error based on future worldwide experimental series.