The environmental degradation of synthetic polymers is enhanced by blending them with biopolymers such as starch, pro-oxidants (metal complexes, such as cobalt stearate and manganese stearate) and inorganic fillers. Recent studies have shown that nano-clay used to enhance the mechanical properties of polymers such as polyethylene also helps in polymer degradation (Lee et al., 2005; Qin et al., 2003). The present work aims to investigate the effect of montmorillonite nano-clay (MMT) on the photo-oxidation of polyethylene. The molecular weight changes of polyethylene in MMT/polyethylene nanocomposites upon UV irradiation were evaluated using rheological measurement and gel permeation chromatography. The molecular weight decrease for nanocomposites is found to be greater than that for the pure polyethylene. Furthermore, the increase in clay concentration also enhances the rate of photo-oxidation of the polymer. These results show that clay is acting as a self pro-oxidant during the photo-oxidation. The dynamic shear measurements show that both cross-liking and chain scission occur during the photo-oxidation but the latter seems to be dominant.