This paper describes an investigation into the ventilation performance and the indoor air quality of a portable classroom. Both field measurements and numerical simulations based on CFD (Computational Fluid Dynamics) technology were used. Field measurements in an unoccupied classroom used smoke to visualize the flow pattern, and hot-film probes to quantitatively measure air velocity. These field measurements provided the boundary conditions for CFD simulations and the experimental data to examine the accuracy of the CFD simulations.