The crystallization process and the electrical properties of amorphous tin-doped indium oxide (ITO) films have been studied in contrast with those of undoped indium oxide (IO) films. Amorphous ITO and IO films were prepared by magnetron sputtering succeeded by annealing in the air at various temperatures. ITO films showed higher crystallization temperature compared with that of IO films, suggesting an excess free energy caused by the repulsion between the active donors ($Sn^{4+}$). The analysis of the electrical properties alternated with the phased annealing of films provided essential information for understanding the conduction mechanisms of ITO. It was also revealed that the amorphous IO/ITO films showed oxidation around $100^{circ}C$ in contrast with crystalline IO/ITO films with the oxidation temperature above $200^{circ}C$.