Continuous UV irradiation on textiles has been recently exploited as a practical surface treatment due to the commercial development of high intensity UV light source. In this study PET and PTT fabrics were continuously irradiated with an electrodeless UV lamp for different periods of time. The effect of UV/O$_3$ treatment on the mechanical properties of the polyester fabrics relevant to the apparel manufacturing process, such as fabric thickness, crimp, modulus, extensibility, bending and shear rigidity, were studied. The physical properties of PTT fabric were more influenced by the UV treatment than PET fabric. The widthwise extensibility of treated PTT fabric increased with UV/O$_3$ irradiation time due to crimp increase and fabric shrinkage which occur during UV/O$_3$ irradiation. The shrunken fabric structure resulted in increased bending rigidity. However the shear rigidity of treated fabrics decreased due to stress relaxation, presumably by infra-red emitted with UV irradiation.