Abstract: Occupants' behavior of operating the air conditioning unit plays a significant role in energy conservation in buildings． A field monitoring was carried out to investigate the air conditioner usage behavior in an office building located in Hangzhou in summer． The probability of air conditioning unit was turned on and the great indoor thermal environment difference with the different air conditioning unit in operating state showed that the air conditioning unit played the key role in indoor space cooling in summer． The logistic regression was used to develop a statistical model for air conditioner usage behavior during summer． The outdoor tolerant temperature was found as 32. 0 ℃ based on the proposed formula in the logistic regression，the indoor tolerant air temperature was found as 28. 7 ℃．