A questionnaire survey was conducted to investigate the thermal environment experienced by young children in both summer and winter. The questionnaire consisted of two parts; the basic and the fact-finding questionnaires. The first part was designed to reveal the detailed characteristics of environmental preferences for both young children and their mothers as well as mothers' behavior regarding thermoregulation and their living environments. The second part sought information regarding the measurement of room temperatures, the activities, clothing and mothers' thermal sensations at home. The second part including the measurement of room temperature was conducted four times (9-10 _AM, 1-2 _PM, 5-6 _PM, 8-10 _PM) on a chosen day in winter as well as in summer. The results are as follows. 1) Over 90% of the case, use of the heating and cooling equipment depended on the mother's thermal sensation in winter, while, 50% of the case, mother's thermal sensation led the behavioral thermal regulation at home in summer. The choice of children's clothing depended on weather (80%), mother' s thermal sensation (40%), and room temperature (20%). 2) The field survey showed that the average ambient air temperatures in the dwelling where mothers and their children lived were 18°C in winter and 28°C in summer. 3) The young children's clothing was one or two pieces fewer than mothers' both in summer and winter. The average clothing insulation for mothers was estimated to be 0.35 clo and 1.05 clo in summer and winter respectively, while the clothing insulation for children was estimated to be 0.1 clo and 0.3 clo less than that for mothers in summer and winter respectively. 4) The relationship between ambient air temperatures and the mothers' thermal sensations was good in both winter and summer. The neutral temperatures for mothers that were determined from respective liner regression were 16°C and 26°C in winter and summer respectively. The other neutral temperatures determined by mothers' clothing insulation were 21°C and 27 °C in winter and summer respectively.