Nylon 66 films were photo-irradiated at 60 degrees C for different time intervals up to 200 h. Photo-irradiated samples were characterized by XRD, DSC and optical microscopy. XRD results showed that there was increase in crystallinity with increase in exposure time during initial period of photo-exposure. However, there was no change in X-ray diffraction pattern upon photo-irradiation. DSC results showed that there was formation of new crystalline morphology upon photo-irradiation. This formation of new crystalline morphology was observed to be at the expense of existing morphology. Optical microscopy showed that there was formation of cracks at the center of spherulites as photo-irradiation proceeds. Cracks were increased in size with photo-irradiation.