Nanocomposites of polypropylene-graft-maleic anhydride/clay were prepared by melt blending in an extruder mixer. The nanoscale dispersion of the clay in the polymer was analysed by wide-angle X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results of XRD and TEM showed that the nanocomposites obtained were a kind of intercalated-delaminated structures side by side with different dominant states, depending on the clay used and on the processing conditions. The consequences of photo-oxidation on the thermal stability and fire retardant properties of the nanocomposites were investigated using thermogravimetric analysis and cone calorimetry tests. It appeared that this degradation dramatically affected the important properties of the nanocomposites. A loss of thermal stability and fire retardant performance was observed. This was ascribed to scission reactions that occurred during the oxidative degradation prior to thermal and fire tests.