The change of warmth retaining properties of non-woven fabric with thickness and air velocity has been studied. The transmission of air velocity was approached to inverse proportion at above thickness 30 mm in air velocity 1.5 m/sec but in the above air velocity that was disregarded. The effect of air velocity could be indicated to time of reach thermal equilibrium and transmission of air velocity. In the thermal equilibrium state, transition area which has less influence on the air velocity was detected in sample thickness 30 mm to 50 mm.