Effects of air velocity and packing density under controlled heating and constant heat flow on thermal insulation of nonwoven fabrics were studied. Results obtained can be summarized as follows; 1. Thermal insulation should be considered in terms of air velocity, packing density, time required to reach thermal equilibrium and thermal behavior. 2. Time for thermal equilibrium decreases with increasing air velocity and decreasing packing density, respectively, while it increases with increasing packing density. 3. Thermal insulation is fairly sensitive to increasing packing density at the air velocity of 0.5m/sec while it becomes less sensitive to increasing air velocity, representing almost constant values at the air velocity of more than 2.0m/sec. 4. Thermal insulation effeciency of nonwoven fabrics makes than 10 percent, providing unfavorable thermal insulation effect.