Emission characteristics of polymer light-emitting devices (PLED) with a configuration of indium-tin-oxide/poly(3-dodecylthiophene)/semitransparent-Al changed drastically upon photo-irradiation in air. After photo-irradiation to PLED, current passing through the device decreased by about one order of magnitude and emission disappeared within several minutes. Patterned emission from PLED utilizing this effect was also demonstrated. The observed modification should originate from slight photo-oxidation at the polymer/Al interface and/or the polymer layer. The optical patterning method mentioned here can be applied onto the completed devices, in contrast to other methods such as ink-jet printing technology, in which the patterning of the polymer must be carried out before deposition of the metal electrode.