Human thermal comfort in car cabin has become an important factor to evaluate the success of HVAC (heat ventilation air condition) system. To a HVAC of high quality, it is necessary to produce proper temperatures and air flows at all points including the human body, not just for some specific design conditions. But the current situation is that numerical simulation of HVAC system has still lacked of the link with the thermal source of human body, because of the difficulties in modeling the extremely complicated fluid/thermal system of human body. In this paper, a HVAC system with an accurate thermoregulation model of human body has been simulated in SC/Tetra CFD software, which successfully solves the critical problem coupling the air flow around the body with the human body internal heat transfer physics. Graphical output (including air velocities and temperatures) from SC/Tetra and JOS shows how a particular automotive HVAC system performs. The skin temperature computation got from JOS accounts for all the factors affecting human body heat transfer such as radiation from the sun, conduction where bodies contact seat, human individual physiological makeup and so on. These results can be used as a thermal comfort index to quantify the effectiveness of a HVAC system, to evaluate design modifications and to take a major step toward improving the HVAC system.