To clarify an effect of polymer chain scission on a polystyrene (PS)/multi-wall carbon nanotube (MWNT) composite photodegradation, a relationship between the change of molecular weight and photodegradation behavior was studied. The MWNT loading brought about severe PS chain scission and led to the increase of the low molecular weight (less than 105) fraction. The increase of the fraction was not proportional to the loading amount and showed the minimum at the 2% loading. The strange behavior was due to a rheological effect bringing about a decrease of shear stress in the composite preparation. An unsaturated end group was produced by the chain scission and became the photodegradation initiator leading to auto-oxidation and crosslink reactions. The MWNT scavenged radical species and worked as an antioxidant. The coexistence of the unsaturated end group and MWNT made the photodegradation behavior complicated. However, the MWNT radical scavenging ability was considerably poor, and the MWNT had little ability to inhibit the photodegradation initiation. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40362.