Interfacial friction in the core affects the two-phase mixture level and the distribution of the dispersed gas phase during a small-break loss-of-coolant accident (LOCA). The RELAP5/MOD3.2 code uses the drift flux model to describe the interfacial friction force in vertical dispersed flow, and the Chexal–Lellouche drift flux correlation is used for the rod bundle geometry. In the present study, the RELAP5 model uncertainty was quantified for the bubbly–slug interfacial friction model in the rod bundle geometry by conducting numerical analyses of void profile tests in the Thermal Hydraulic Test Facility (THTF) of the Oak Ridge National Laboratory (ORNL). The model uncertainty parameter was defined as a multiplier for the interfacial friction coefficient. Numerical analyses were performed by adjusting the multiplier so that the predicted void fractions agreed with the measured test data. The resultant distribution of the multipliers represented the interfacial friction model uncertainty.