We analyzed the temperature distribution in low temperature poly-Si thin film transistors using infrared thermal imaging microscope, and discussed the thermal degradation by Joule heating. A non-uniform temperature distribution was observed in the saturation region along the gate length. Increase of temperature was remarkable for wide gate widths. which yielded large threshold voltage shifts, A universal relationship was obtained independent of the crystallinity of poly-Si films, which suggested that we should take into account the degradation of the gate oxide for example by electron trap generation. (C) 2005 Elsevier B.V. All rights reserved.