Beneficial structural properties of weft knitted fabrics renders them to be used in wide spectrum of general end-uses such as apparels, technical fabrics and particular end-use of sport-wear. Transfer properties of weft knitted fabrics such as air permeability in most of these applications is of paramount importance. This work aims to investigate air permeability of interlock fabrics using experimental and numerical simulation methods. Samples of interlock fabrics at three loop lengths were knitted using 100 and 150 diner polyester multi-filament yarns. Air permeability of samples was investigated using experimental method. In order to simulate air flow through the samples, the structure of interlock fabrics was modeled using virtual simulation and X-ray micro-computed tomography ( µ CT). In the virtual simulation method, the 3D geometry of loops was modeled in the CATIA software and fluid flow was simulated using finite volume method in Fluent software. In µ CT method, the realistic geometry of 3D loops was obtained using µ CT imaging and fluid flow was simulated using finite volume method in Avizo software. Sharp reduction in stitch density of both set of yarns was observed due to increase in loop length from the level 1 to 2. However results point to less reduction in stitch density when the loop length is increased from the level 2 to 3. It was found that post-knitting treatment given to the samples leads to increase in stitch density of the fabrics. It was concluded that reduction in stitch density due to increase in loop length results in increase in voids in the fabric which in turn results in higher air permeability. This trend was found to exist for samples knitted with the both types of yarns when the loop length was increased from level 1 to 2. However, results pointed to the reduction in fabric air permeability when the loop length was increased from the level 2 to 3. This was attributed to the significant increase in fabric thickness despite observed reduction in stitch density of the samples. Error percentage in virtual and µ CT methods was found to be 10-16 and 3-5 percentages, respectively. It was concluded that both methods used in this work can most satisfactorily be used in prediction of weft knitted fabrics air permeability.