The present study investigates thermal mixing and stratification produced by heated air jet located at the bottom level of the containment vessel. The experiment was carried in the large containment vessel called CIGMA (Containment InteGral effects Measurement Apparatus), and the numerical analysis was performed using the data of those experiments. The experiments were conducted with external surface cooling and various air jet inlet temperatures. The containment cooling was done by flooding with water the external side of the upper half of a vessel. To identify their influence on the thermal mixing and stratification phenomena, the investigation focuses on mixing convection, which occurred in the cooled region of the containment vessel. Temperature distribution and jet velocity were measured by thermocouple and Particle Image Velocimetry (PIV), respectively. Numerical simulation was performed using Computational Fluid Dynamics (CFD) code OpenFOAM to investigate the detailed effects of external cooling on the fluid flow and thermal characteristics in the test vessel. CFD results showed a good agreement with experimental data on both temperature and velocity. Both temperature and velocity of hot air jet decayed rapidly downstream jet nozzle. Thermal stratification was observed by visualization of temperature contour maps over a cross-section in the containment vessel. Vigorous mixing was also noticed in the upper region of the containment vessel. Effect of external cooling on mixing and the thermal stratification were presented and discussed.