The water vapor permeability of polypropylene spun bond non-woven fabrics were investigated with the water vapor permeability velocity at 20℃, 30℃ and 40℃ by applying the hygroscopic method. At each temperature 50, 65 and 80 %RH conditions were used. The results indicated that the water vapor permeability velocity increased with increasing the water vapor concentration difference between both sides of sample surfaces and it decreased with increasing the number of the piled-up fabrics and the apparent density.