A long term field study was conducted in an office building in Guangzhou, which is located in the Hot Summer and Warm Winter climate zone of China. The study covered three seasons – summer, autumn, and winter. Physical environmental parameters were continuously recorded, while office workers filled in thermal comfort related questionnaires once a week. In summer, cooling was provided to the investigated rooms with relatively high setting temperatures. The neutral temperature of occupants was 26.8 °C, and the upper limit of the acceptable temperature range was 30.4 °C. Unless the indoor temperature was higher than 29 °C, more than 90% of people could accept the thermal conditions, which proved the adaptation to warm environment of local people. At a same outdoor temperature level, higher indoor temperature resulted in less energy consumption through air conditioning system in summer. Thus, the temperature of air-conditioning in office buildings in hot climates could be set to a higher level than it usually was, so that air conditioning energy could be saved while occupants’ comfort won't be sacrificed. In autumn and winter, no heating or cooling was provided to the investigated rooms, and the occupants’ neutral temperatures in the two seasons were 25.8 °C and 23.3 °C respectively. Compared to the PMV/PPD based evaluation method in ASHRAE 55 standard, the adaptive thermal comfort model based method fits better with the occupants’ subjective thermal acceptance.