The intensity distribution of the ion flux on a substrate surface has been measured to examine the origin of the inhomogencity of resistivity in magnetron-sputtered indium tin oxide (ITO) thin films. Ion flux intensity was determined as the product of ion saturation current I(i) and floating potential V(f) which were measured by a cylindrical Langmuir probe placed near a glass substrate. I(i)V(f) varied depending on the substrate position and showed a maximum near the upper position of the target erosion line. The resistivity of the ITO films showed a minimum at a certain oxygen flow rate. The oxygen flow rate where resistivity showed a minimum decreased as I(i)V(f) increased. Carrier mobility increased as I(i)V(f) increased under low oxygen flow rate conditions. Carrier concentration decreased as I(i)V(f) increased under high oxygen flow rate conditions. The film microstructure and composition were related to the electrical properties.