This paper presents the results of field experiments and the CFD simulations on the ventilation efficiency in an underground car park where several high velocity fan-diffiiser units were installed. In this study the age of air and the local air change effectiveness were evaluated with the field experiments and the effect of the high velocity fan-diffuser unit on the ventilation efficiency was examined. Then the CFD simulation was carried out with the same condition as the field experiment and the simulated results were compared with the experimental one. In the CFD simulation, the residual lifetime of air and the residence time of air were also calculated to study the effect of the high velocity fan-diffuser units. The results of this paper are following; (1) There were no remarkable effects of the high velocity fan-diffuser unit on the age of air in the underground car park from both the experiments and the simulations. (2) The simulated results showed good agreement with the experimental one. (3) From the residual life time of air calculated with CFD simulation, it was found that the high velocity fan-diffuser unit was effective to activate the stagnant air in the parking area enclosed with the attached room and the wall, etc. (4) The improvement of the ventilation efficiency can be expected with the combined system of the piston-flow in the drive-through area and the high velocity fan-diffuser units in the parking area