For air conditioning systems in buildings, thermal comfort and energy saving should be simultaneously considered. This paper describes the thermal comfort control for the indoor thermal environment. The proposed method is a fuzzy PID controller based on the least enthalpy difference. The minimum enthalpy difference estimator (LEE) calculates the indoor dry bulb temperature and relative humidity to produce the optimum temperature setting values as input of the controller. According to the temperature setting values, the fuzzy PID controller based on the least enthalpy difference can satisfy the requirement of thermal comfort control. In this paper, the correlation between the relative humidity and the temperature setting values is discussed.