Semiconducting tin oxide thin films with suitable catalysts in the form of nanoparticles, overlayers and clusters are known to exhibit enhanced sensitivity, better selectivity and fast response speeds to various reducing gases. SnO2 sensor is invariably anion deficient and oxygen vacancies are mainly responsible for making available free electrons for the conduction process. Fe3+ doped tin oxide thin films were prepared by chemical spray pyrolysis. The prepared thin films were characterized by electrical, gas sensing and thermo emf. The thermo emf of Fe3+ doped SnO2 thin films increased with the increasing of temperature. At low temperatures the Seebeck coefficient is observed to be high and the Seebeck coefficient decreases with increasing of temperature. The large values of thermoelectric power of Fe3+ doped SnO2 thin films are typical of semiconductor behavior.