The objectives of this study are to clarify the air flow mechanism through fabrics and also to obtain an equation describing the air permeability of fabrics. A theoretical equation was obtained using a simple model of a fabric air flow mechanism in which the effects of both the intra-yarn and inter-yarn interstices were taken into consideration. Some parameters in the equation could be rewritten into functions using basic fabric parameters from a viewpoint of the practical application to fabric performance design. By such a simplification of the parameters, a prediction equation using fabric thickness, yarn density, yarn count, fiber density and fiber diameter was obtained. Some examples of the calculation according to he equation show how fabric parameters such as yarn density, fiber diameter, fiber type etc. influence the air permeability of fabrics. The adequacy of the equation will be verified in another paper.