The idea of In-Vessel Retention (IVR) has been considered as a feasible technique to keep the reactor pressure vessel (RPV) integrity in case of a severe reactor accident. For the LWR, the effectiveness of this strategy relies soundly on the critical heat flux (CHF) distribution over the external surface of RPV lower plenum, whose orientation angle varies gradually from heading downward horizontal to the vertical position. The CHF prediction capability of the liquid sublayer dryout model is efficient for high mass flux and inlet subcooling on vertical subcooled flow boiling conditions. This paper focuses on the research on bubble departure diameter (dB) and net vapor generation point (NVG) to assess the predictive potential of CHF under IVR conditions. An experimental facility was designed to acquire the data of bubble departure diameter and NVG for different orientation angle conditions of heating surface (from downward facing horizontal to vertical). A modified liquid sublayer dryout model is proposed where the orientation effect is included to measure the bubble departure diameter (vapor blanket diameter) using an improved force balance model, and the NVG point is modified according to the departure diameter. The predicted diameter and subcooling at NVG generally show good consistency with the measured values, and the modified liquid sublayer dryout model can predict the experimental CHF data within ± 20% in IVR conditions.