This paper presents a comparative study based on CFD simulation between the performance of Impinging Jet Ventilation (IJV) and Mixing Ventilation (MV) systems in providing indoor air quality and thermal comfort for a mechanically ventilated occupied large open plan office (floor-to-ceiling height > 5m). Large spaces differ from spaces with standard heights because of the significant upward stratification. The evaluation was carried out using the Air Distribution Index (ADI) which combines several parameters, such as overall ventilation effectiveness for removing pollutants and for temperature distribution , percentage of dissatisfied (PD) and predicted percentage of dissatisfied (PPD). ADI has been used to characterize ventilation systems in spaces with typical floor-to-ceiling heights, but it has not been studied for large spaces to-date. In this study, different test conditions were considered for two cooling loads (48 W/m2, 68 W/m2) with full and half occupancy respectively. The model was validated via spot measurements of air temperature, air speed and CO2 concentration in a large office located in south east England which, is supplied by a mechanical cooling with overhead diffusers providing mixing ventilation. The predicted results were generally in good agreement with measured values of air temperature, air velocity and CO2 concentration. The calculated (ADI) for (IJV) and (MV) systems using the validated CFD model showed that the (IJV) system is more effective than (MV) system in removing both pollutants and heat from the occupied zone. It is concluded that using (ADI) to assess the effectiveness of different ventilation systems in large enclosures can provide useful information that combines both indoor air quality and thermal comfort.