Tin Oxide films of less than 30nm in thickness were developed as transparent conductive electrodes. The films were deposited onto glass substrates by APCVD. Fluorine was used as a doping component. Stability to heat treatments in air at more than 500• was studied. Carrier concentration decreased and Hall mobility increased by the heat treatments. It was found relations between carrier concentration and mobility exhibited an exponential relation in extremely low fluorine concentration films. The exponent of the relation was close to −1.5. Resistivity decreased for the films, while it increased for films with high fluorine concentration. It was also found migration of sodium from the substrates into the films increased with increase of fluorine concentration in the films. Results suggest the sodium migration would affect grain growth and electrical properties of the films.