The In2O3 thin film nanosensors were fabricated by dropping In(OH)3 sol on nano gap electrode with 0.1-2.5 micron of gap size, dried, and calcined at various temperatures. The grain size of In2O3 and thus the number of In2O3 grains were changed by calcining at 600-850 °C. The size of grain was increased from 15 nm at 600 °C to 55 nm at 850 °C, and thus the number of grains in 0.3 micron gap was decreased from 19 to 5.3. Contrary to the expectation, the small resistance changes to both 1 ppm Cl2 (resistance increase) and 0.5 ppm Cl2 (resistance decrease) were obtained for In2O3 nanosensors calcined at 700-850 °C. These were contributed to the changes in the surface and interface chemical state to suppress the adsorption of or substitution with Cl2 molecule.