Air permeability of a fabric is defined its the amount of air, passed over a surface under a certain pressure difference, in a unit time. This value has a significance with respect to the usage area. Since knitted fabrics have a loop structure, they have more pores than woven fabrics, so, in general, air permeability of knitted fabrics are higher than the woven fabrics having same weights. The experiment of determination of air permeability that defines the properties of keeping warm, protection against the wind, breathability etc. of knitted fabrics used as clothing, is very important.