Photooxidation degradation of polypropylene (PP)/organomontmorillonite (OMMT) nanocomposites was studied under UV irradiation at wavelength 365 nm. The OMMT was dispersed homogeneously in the PP matrix. The structural variation induced by photooxidation was detected by FTIR spectroscopy. It can be seen that the evolution of PP and nanocomposites PP/OMMT on irradiation is an increase in absorbance in the hydroxyl and carbonyl absorption regions. Carbonyl index was used to characterize the rate of photooxidative degradation. At the early stage of irradiation (shorter than 11 hours),the rate of photooxidative degradation of PP/OMMT nanocomposites was much faster than that of pure PP. With the increase in the irradiation time,the rate of photooxidative degradation of PP/OMMT nanocomposites became slower than that of pure PP. On the other hand,the OMMT content can also influence the photooxidative degradation rate. The variation in the photooxidative degradation rate is proposed to originate from the competition between the light shielding effect and accelerating photo-oxidative degradation effect of organomontmorillonite. These two effects play roles in the photooxidation process of PP simultaneously. At the early stage of irradiation,the accelerating photooxidative degradation effect dominates. With the increase in the irradiation time,the active sites such as SiOH, —OH etc and transition metal ions (Fe2+,Fe3+ etc) in the OMMT which can accelerate photooxidation are gradually consumed,and then the light shielding effect may play the main role. The method of curve-fitting was used to characterize the influence of organomontmorillonite on the photooxidative degradation products. Compared with pure PP,the content of products of carboxylic acid and anhydride were found to increase, whereas that of ester decreases in PP/OMMT nanocomposites.