Indium tin oxide (ITO) films are well known as a transparent conducting film and used for various display devices. Their conducting characteristics are contributed by free electrons arising from four valent doping Sn substituting in the In lattice and doubly charged oxygen vacancies. The structure of ITO films can be changed from amorphous to crystal phase depending on the process conditions. In this report, to get further insight into the conducting properties of ITO films, the in-situ resistivity measurement of various structures of ITO films was performed on annealing in a various flowing gas atmosphere (air, O@sub 2@, H@sub 2@). It was found that an abnormal increase of restivity showing a sharp peak was observed at 220-250@degree C@ for the fully and partially amorphous ITO. The temperatures at the peak were shifted to a higher range in the order of air, H@sub 2@, and Osub 2@flowing gas atmospheres. The Hall measurement revealed that the carrier density and Hall mobility both sharply decrease at the resistivity peak. The activation energy of the abnormal resistivity change was measured to be 0.67-1.7 eV by using the Kissinger's method. The activation energy thus measured are different depending on the annealing atmosphere. The X-ray diffraction profiles and TEM observations revealed that the crystallization took place at ~180@degree C and continuously progressed with further increase of annealing temperature. Note that any further visible structure changes were not observed after the peak resistivity. Taking into account the fact that the activation energy of an oxygen vacancy in an In oxide is reported to be about 5eV which is much higher than the present measured values, and also an In in amorphous ITO films is reported to play a large role of the amorphous to crystalline phase transition at ~150@degree C@, the realignment of Sn-O bond to generate a locally ordered structure most likely responsible for the present abnormal resistivity change observed.