Thermal resistivity is an important factor in the body-clothing-ambient system to provide protection of body from the environmental condition. This paper intended to investigate the relationship between thermal resistivity of the body-clothing-ambient system and the thickness of the air spaces in the system. For this investigation, the authors made a testing apparatus of a cylindrical heater of constant temperature to simulate the body-clothing-ambient system. Five cotton plain fabrics of different fractional fabric cover and their P.V.C. film coated fabrics were used for specimens. Specimens were shaped a cylinder form to simulate the sleeve of cloth and heat losses of the cylinder covered by specimens were measured. The experimental results were converted into clo unit, and the relation between thickness of air spaces in the system and total thermal resistivity were analysed. The conclusion of this study were followings: 1. For good thermal resistivity, it is better to make air spaces in the body-clothing-ambient system. 2. The maximum thermal resistivity was shown when the thickness of air space between the body and clothing or between clothings was 16mm. 3. When total thickness of the air spaces in the body-clothing-ambient system was constant, the maximum thermal resistivity was shown when air spaces in the system were equal. 4. When the thicknesses of each air spaces were equal, the equation for thermal resistivity became as follow. Y=0.21658Z＋0.1150630e-X, x0.016－679.9(x-0.016)2＋0.14108 Y: stands for thermal resistivity in clo. X: stands for air space in m. Z: stands for fractional fabric cover.