ITO thin films have been prepared by DC magnetron sputtering on a carbon film at room temperature. The fine structures of as-deposited films and also of those after heat treatment at 473 K in a vacuum have been observed by an electron microscope. The as-deposited film is mostly amorphous and includes some round shaped nuclei in it. The nucleation takes place on the substrate surface and its orientation is random. By the heat-treatment, each crystal grows without taking the crystal habit before the entire film is completely crystallized into fine grains. The crystal orientation of the grains is random after the crystallization is completed or even after the recrystallization of grains takes place.