Thermal degradation of indium tin oxide (ITO) thin film has been investigated. ITO thin film was fabricated on glass substrate using RF magnetron sputtering and was characterized. The resistivity of the film which was thermally degraded at high temperatures in air atmosphere was increased highly. Thermally-degraded specimen was analyzed using XPS and Hall measurement to reveal failure mechanism. Result showed that failure mechanism was the decrease in charge carrier concentration and mobility due to oxygen diffusion and chemisorption. Accelerated degradation test (ADT) was performed to predict the lifetime of ITO thin film. The lifetime under normal operating condition could be predicted via statistical analysis and modeling of data acquired from ADT of a short period.