In order to fond the relations between the warmth retaining property and the thermal behavior of fiber assembly, we studied the warmth retaining properties by supplying 50kca1/m2h of heat, which is the same amount of heat released from human skin. Applying a numerical analysis of finite-element method, we examined the thermal behaviors of the fiber assembly by changing both the air content ratio and the thickness of the sample. The results were as follows. When the sample thicknesses were thicker than 20mm, the thermal equilibrium times were found to be effected greatly by the variation of the air content ratio. However, when the sample thicknesses were less than 20mm, it was found that the thermal equilibrium times ware essentially independent of the air content ratio. The temperature gradients of the thermal behaviors showed increasing tendencies near the heat source. When the air content ratio was flexed at 40%, the effective heat conductivity was kept decreasing with the thickness even at the thicknesses of 10mm and mote. However, at the air content ratio of 60%, the effective heat conductivity decreased with the thickness until the thickness became 10mm and became almost constant with further increase of the thickness.