The relation between the decrease in resistivity and crystal growth of an indium-tin-oxide (ITO) thin film which was prepared on a substrate at room temperature and then heated from room temperature to similar to 533 K in a vacuum was studied. Two steps of resistivity decrease on heat treatment at around 383 K and 493 K were observed. That at the lower temperature took place while keeping the amorphous state, being due to a small increase in carrier concentration and also a slight improvement of Hall mobility. The other decrease at the higher temperature is due to crystallization from the amorphous state, which gave rise to the great increase in carrier concentration.