The chitosan treatment of fabric involves dipping in highly viscous chitosan solution. One of the anticipated drawbacks of the dipping of the fabric samples in highly viscous chitosan solution would be poor air permeability by clogging the pore structures of the sample. There have been no quantitative studies on the changes of air permeability with the treatment conditions. In this study, the effect of molecular weight of chitosan on the air permeability and the effect of chemical nature of fiber materials comprising fabric samples were investigated. Image processing techniques were employed for accurate evaluation of the pore area and the pore size distribution of the fabrics, which could explain the change of air permeability of the chitosan treated fabric samples. The results can be summarized as follows : 1) Chitosan treatment of the fabrics composed of the cellulosic, silk, and wool fibers increased the air permeability, whereas chitosan treatment did not change appreciably the air permeability of PET, nylon, and acrylic fabric samples. 2) High molecular weight chitosan was more effective in improving air permeability than low molecular weight chitosan for the fabrics composed of natural fibers. 3) As chitosan treatment not increases the crispness of the fabric but also improves air permeability for some fabrics, it is a recommendable treatment for fabrics requiring cool touch.