Tin-doped indium oxide (ITO) films were fabricated using dc magnetron reactive sputter deposition of an In 10 wt. % Sn alloy target in an Ar and O-2 gas mixture. To understand the relationship between the electrical properties of ITO films and O-2 partial pressure, the resistivity, carrier concentration and mobility, film density, and intrinsic stress in the films were measured as a function Of O-2 partial pressure. It was found experimentally that an ITO film having both maximum conductivity and carrier mobility has a film density close to its theoretical density. For the ITO film with density close to theoretical, the mean free path was the same as the grain size (=columnar diameter). This indicated that the electrical characteristics of the ITO films discussed here depend strongly on the grain size. However, for the ITO films with density lower than theoretical, the mean free paths were smaller than the grain size. It is suggested that electron scattering at pores and voids within the grain is the major obstacle for electron conduction in the ITO films having a lower density. The measurements of intrinsic stress indicate that the density of ITO films is determined by bombardment of oxygen neutrals on the growing film.