In the paper, conductive and transparent indium tin oxide (ITO) films have been prepared by reactive magnetron sputtering using powder target of In2O3 with 10wt% SnO2 in an Ar/O2 atmosphere. The structural, composition, optical and electrical properties of the films deposited at different substrate temperature (150 approximately 350 degrees Celsius) have been investigated. Some interesting results obtained in this work, which might be somewhat new, are that the variations of oxide structure and concentration greatly influence the electrical properties of ITO films. X-ray diffraction patterns and X-ray photoelectron spectroscopy (XPS) show that the films deposited at substrate temperature 250 degrees Celsius or more are consist of In2SnO5 and In2O3 with no Sn3O4. When substrate temperature is lowered to 150 degrees Celsius, Sn3O4 occurs besides In2O3 in the films. The above result indicates the occurrence of Sn3O4 is unfavorable to the conductive properties. When substrate temperature is increased, Sn3O4 disappears, and oxygen vacancy and the electron donor Sn+4 concentration are increase, which result in a great improvement of the electrical and optical properties.