This study aimed to numerically investigate the air flow and contaminant dispersion in a ventilated room. The URANS equations supplemented with energy and concentration equations are solved using the scStream code. We are targeting primarily the ventilation effectiveness (εc) and the index of indoor air quality (IIAQ). Different displacement and mixing ventilation modes are investigated in isothermal conditions. The ventilation flow rate is 50 m3.h-1 Here, our ultimate aim is to suggest an effective ventilation that can ensure a good indoor air quality. The RNG k − ε model was adopted to handle the turbulence. CFD simulations were systematically checked, whenever possible, through available results. The simulation results indicate that the ventilation effectiveness to remove a contaminant and the index of indoor air quality are substantially influenced by the ventilation mode and the location of inlet and outlet air. From this study, it appears that the air flow and contaminant dispersion in a ventilated room can be typically predicted or evaluated by computer simulations.