Transparent conducting tin oxide thin films have been deposited by spray pyrolysis technique. The effect of substrate temperature on the structure and optical properties of tin oxide thin films has been studied. The increase in the substrate temperature led to increase the optical transmission. A transmittance value of 84% has been obtained for spray-deposited film at 400 degrees C. The absorption coefficient which is used to determine the optical band gap, real dielectric constant and free carrier concentration has been calculated using four equations. An increase in the band gap and the free carrier concentration were observed by increasing the substrate temperature during deposition from 200 to 400 degrees C.