Photooxidation effects on the electroluminescent properties in a poly(p-phenylene vinylene) derivative MDOPPV have been studied. The polymer light-emitting devices (PLEDs) used have a semitransparent Al cathode to make the photodegradation rate of the polymer film in the device comparable to that of a naked polymer film. Upon photoirradiation in air, the external quantum efficiency of the device remained constant, while the emission intensity at a constant voltage rapidly decreased. On the other hand, under similar photoirradiation condition, an MDOPPV film, whose thickness in the same as the polymer film in PLED, showed rapid bleaching of the optical absorption corresponding to π-π^* transition as well as rapid decrease of the photoluminescence quantum efficiency. These results suggest that the photodamaging of carrier transport/injection properties of the polymer plays a key role in the photodegradation of PLED based on MDOPPV, rather than the photocreation of defects acting as exciton quenchers.