In summer the hot outside environment often increases the air temperature in a truck cabin to a high level, which makes drivers very uncomfortable. It is often reported that the concentration of VOCs which is unhealthy for drivers in truck cabins will tend upward caused by high-temperature environment. At the same time, the air-conditioning load for cooling truck cabins is large. In this study, a double-ceiling ventilation system for trucks is presented and the effects on thermal environment improvement for truck cabins are discussed by CFD analysis. [This abstract is not included in the PDF]