The natural photo-oxidation of PP bars containing nucleating agent (TMB-5) were studied by using infrared microscope (IM), differential scanning calorimetry (DSC), scanning electron microscope (SEM) and polarizing optical microscope (POM). The results indicated that nucleating agent accelerated the oxidation of PP significantly. The oxidation began from the surface and developed into the interior with time. There is a transition region near the surface, caused by the fast cooling during the injection moulding. This region is prior to be oxidized because of the low density. Therefore, it corresponded to the surface oxidation layer in PP and PP/TMB-5 bars. The nucleating agent helped to reduce the spherulite size of PP and broaden the transition region greatly – from about 25 μm to more than 200μm, covering the morphology of amorphous, fine crystals to relatively perfect crystals. More crystal boundaries, i.e. low density, and broad transition region led to higher oxidation degree and further oxidation along the depth. However, the thickness of the surface oxidation layer maintained nearly constant till the fragments dropped from the seriously damaged surface.