Crystalline-orientation-dependent carrier density in sputter-deposited tin-doped indium oxide (ITO) films was discovered by the epitaxial growth technique. Despite identical Sn concentration, (111)-epitaxial films always showed a lower carrier density compared to films of other crystalline orientations. A crystallographic investigation of the ITO material revealed that the crystalline growth along < 111 > accommodates the larger amount of interstitial oxygen atoms (O-int). These atoms reduce the density of free electrons when combined with substitutional tin atoms which act as donors in ITO material. Thus the peculiarity of the electrical properties of(111)-epitaxial films is attributed to the crystalline orientation dependence of the population of O-int.