This paper presents and discusses measuring results of ambient and indoor air temperatures in 144 living rooms or bedrooms of 98 households without air conditioner installed. The automatic measurements of temperatures were conducted by using Rhlog temperature-recorders and lasted nearly a month from 15/7/99 to 9/8/99. The main purpose of these measurements is to get better understanding of thermal performance of existing residential buildings in Beijing. For this purpose, comparison of air temperature distributions between outdoor and indoor both in the whole day and nighttime has been introduced. Analysis shows that the indoor climates of existing residential buildings in summer are far dissatisfied with occupant thermal comfort. However, there still remain a few rooms, in which the highest temperature was only 30.5℃ even when the outdoor air temperature has been higher than 40℃. This indicates that the target of thermal comfort in residential buildings could be achieved by improving building design to a certain extent. Further discussions have also been performed in detail about the averages of indoor air temperatures as well as their amplitudes and time lags. Results show that proper insulation levels and favorable ventilation are the two essential factors for the improvement of thermal performance of residential buildings.