Air conditioning is defined as the process of treating air so as to control simultaneously its temperature, humidity, cleanliness and distribution to meet the requirements of the conditioned space. As in the definition, the important actions involved in the operation of an air conditioning system are temperature and humidity control, air purification and movement. For these conditions this paper proposes a Automatic Climate Control(ACC) system of the bus. The system has cooling, heating, and dehumidifying modes, and is governed by dual 8-bit microprocessors. These modes are broken down into sub-modules dealing with control of the compressor, blower speed, damper position, air purifier, ventilators, preheater, air mixing damper and so on.