The purpose of this study is to examine the effects of body posture on thermal insulation of clothing. Clothing ensembles including skirts were mainly investigated. A new thermal manikin was developed for this study. This manikin was able to change her body posture without increasing or decreasing her surface area. By changing posture from standing to seated, the total thermal insulation of clothing (It) was decreased by around 1.4%. The surface thermal resistance at nude (Ia) was increased by 8%. As a result, the basic thermal insulation of clothing (Icl) was decreased by 14%. The main reason for the decrease of basic thermal insulation (Icl) when the posture of thermal manikln changes from standing to seated was the increase of la at nude. The total thermal insulation of clothing for each body posture (Iti), when standing and seated, were compared. The parts affected by the changed posture were abdomen, hip and thighs, and especially under the skirt of 61cm length, the knees were most affected.