Abstract: The in-vessel retention (IVR) strategy is a key technology to retain the core melt in reactor vessel and prevent the leakage of radioactive material after severe accident happened to the nuclear reactor. The nanofluid is a fluid with steady suspension of solid nanoparticles, in which solid particles with diameters below 100 nm are added to base fluid to improve heat transfer characteristics of working fluids. Its thermal physical properties and heat transfer characteristics are much different from the conventional solid particle suspended working fluid. Thus, the nanofluid with appropriate nanoparticle type and volumetric concentration can enhance heat transfer. In this study, the MPS-MAFL (moving particle semi-implicit method-meshless advection using flow-directional local grid) method was used to simulate the growth, departure and attaching and gas film forming of the bubble on the downward-facing heating surface in pure water and nanofluid (volume fraction is 1.0%Al₂O₃/H₂O) flow process. The critical departure angle of bubble and its influence factors were studied. This research work lays the theoretical foundation of enhancing IVR capacity and accelerating the industrial application of nanofluid.