The purpose of this project is to fabricate an optimum vehicle air conditioning system of a real vehicle for both comfort and windshield defog in rainy days. According to the studied results of NSC project “The Best Defrost Method of Vehicle’s Windshield by Experimental and Computer Aid Analysis ” (NSC-89-2212-E-168-011), while using any one of the present various air conditioning systems of vehicle during the hot rainy days, the driver will use the A/C function to blow toward the passengers for cooling down the air temperature inside vehicle and make passengers more comfortable. The lower the temperature inside the vehicle; or/and the slower the vehicle speed (the lower heating effect on the windshield by the outside hot air); or/and the higher the temperature of outside air; or/and the higher the relative humidity (the higher the dew point of outside air), the higher possibility of causing the fogged windshield will be. Because in the above situations, the outside temperature of windshield will be more possible lower than the dew point of outside air. The only defog method is to open the heating function simultaneously to make the inside temperature higher and then make the outside temperature of windshield higher. But it will cause a higher uncomfortable temperature inside vehicle as well. Thus, driving a car during the hot rainy days, it is very hard to defog the windshield effectively by using the present air conditioning system of vehicle. The car accidents will be always happened because of the slipping road caused by rainy weather and poor eyesight of driver. After studied the various air conditioning systems of vehicle repeat and repeat during the past two years, it has been found that an optimum air conditioning system of a vehicle for both comfort and windshield defog in rainy days can be achieved by using the knowledge of defog to modify the flow tunnels of present air conditioning system. In the meanwhile, wherever cold, warm and hot regions, whenever spring, summer, autumn and winter seasons, this improved system can defog the windshield effectively when driving during rainy seasons. And the most comfortable temperature can also be obtained simultaneously. Additionally, the automatically control air conditioning system of steady temperature and steady humidity as well as windshield defog can be also obtained by using the method and design developed by this project.