This study deals with the effect of fiber morphology such as the diameter, shape of cross-section and fiber crimp on air resistances of various stages of fiber assemblies, that is, randomized fibrous materials, slivers, nonwoven fabrics, parallel arranged yarn assemblies, and woven fabrics. The Kozney-Carman equation was applied to the air permeability of various fibrous materials. But, for randomized fibrous materials and slivers, it was necessary to modify the equation. The modified equation was obtained by using the experimental data. It has been shown that this equation is useful for the case of nonwoven fabrics in which two different fibers each with different diameters are mixed at various percentages. The fiber diameter also influenced the air resistances of yarn assemblies and fabrics. The smaller the fiber diameter is, the higher the air resistance of these fabrics becomes. In addition, it has been shown that the fabric made of crimpier fibers tends to have high air resistance.