Indium tin oxide (ITO) thin films were prepared on alumina ceramic substrates by radio frequency magnetron sputtering. The samples were subsequently annealed in air at temperatures ranging from 500 to 1,100 °C for 1 h. The influences of the annealing temperature on the microstructure and electrical properties of the ITO thin films were investigated, and the results indicate that the as-deposited ITO thin films are amorphous in nature. All samples were crystallized by annealing at 500 °C. As the annealing temperature increases, the predominant orientation shifts from (222) to (400). The carrier concentration decreases initially and then increases when the annealing temperature rises beyond 1,000 °C. The resistivity of the ITO thin films increases smoothly as the annealing temperature increases to just below 900 °C. Beyond 900 °C, however, the resistivity of the films increases sharply. The annealing temperature has a significant effect on the stability of the ITO/Pt thin film thermocouples (TFTCs). TFTCs annealed at 1,000 °C show improved high-temperature stability and Seebeck coefficients of up to 77.73 μV/°C.