Stability is the most crucial issue in the fabrication of oxide thin-film transistors (TFTs) for next-generation displays. We have investigated the thermal distribution of an InSnZnO TFT under various gate and drain voltages by using an infrared imaging system. An asymmetrical thermal distribution was observed at a local drain region in a TFT depending on bias stress. These phenomena were decelerated or accelerated with stress time. We discussed the degradation mechanism by analyzing the electrical properties and thermal distribution. We concluded that the degradation phenomena are caused by a combination of Joule heating and the hot carrier effect.