In this paper, the flow field and the emission field of volatile organic compounds (VOCs) from the surface of indoor building materials in a FLEC (Field and Laboratory Emission Cell) Cavity are examined by 3D CFD analysis. Two types of emission phenomena from building materials are studied here: (1) emission controlled by external diffusion; (2) emission controlled by internal diffusion. The flow field within the FLEC cavity is laminar. With a ventilation rate of 200 ml/min, the air velocity near the test material surface ranges from 0.1cm/s to 1.4cm/s. In the case of the internal diffusion material, with respect to the concentration distribution in the cavity, the local VOC emission rate becomes uniform and the FLEC works well. However, in the case of evaporation type materials, the FLEC is not suitable for emission testing because of the thin FLEC cavity. The diffusion field and emission rate depend on the cavity concentration and on the Loading Factor.