Thermal mixing and stratification phenomena may occur during the loss of a coolant accident or main steam line break accident in the containment of a Passive Containment Cooling System, or in the suppression pools in BWR. However, the present study pays insufficient attention to the thermal stratification phenomena in the containment of small modular reactors(SMR). In this paper, an investigation on the mixing and thermal stratification phenomena caused by the plumes or buoyant jets in SMR containments was carried out. The experiments were both conducted under non-adiabatic and adiabatic conditions for a steel containment. In each condition, two key parameters, inlet temperature, and flow rate were tested by controlling variables to identify their influence on the thermal stratification phenomenon. The visualization experiments illustrated the jet mixing and stratification development. The experiment results were compared with the numerical computation and they reached a good agreement. Thermal mixing and stratification phenomena may occur during the loss of a coolant accident or main steam line break accident in the containment of a Passive Containment Cooling System, or in the suppression pools in BWR. However, the present study pays insufficient attention to the thermal stratification phenomena in the containment of small modular reactors (SMR). In this paper, an investigation on the mixing and thermal stratification phenomena caused by the plumes or buoyant jets in SMR containments was carried out. The experiments were both conducted under non-adiabatic and adiabatic conditions for a steel containment. In each condition, two key parameters, inlet temperature, and flow rate were were tested by controlling variables to identify their influence on the thermal stratification phenomenon. The visualization experiments illustrated the jet mixing and stratification development. compared with the numerical computation and they reached a good agreement .