We prepared ITO films using d.c. magnetron sputtering and investigated effects of oxygen partial pressure on the microstructure and the electrical properties of the films. The ITO films deposited at low oxygen partial pressure showed resistivity of 2 x 10(-4) Omega cm and optical transmittance of about 90%. The resistivity increased as the oxygen partial pressure increased. The preferred orientation of the film was changed as the oxygen partial pressure was changed. The Alms deposited at high oxygen partial pressure consisted of relatively large grains, but those deposited at low oxygen partial pressure consisted of two distinctive features : aggregate of small grains and that of long grains. Possible cause of the observations was speculated using the oxygen vacancy concentration.