Spray pyrolysis technique has been used to deposit Fluorine doped Tin Oxide (FTO) thin films. Optical constants such as refractive index (n), extinction coefficient (K) and the absorption coefficient () of the FTO thin films were determined using spectrophotometric measurement of transmittance, absorbance and reflectance in the spectral range from 172 to 1100nm. The maximum value of transmittance was in the range of 77% to 86%. Furthermore, the effect of annealing temperature and the annealing condition on optical band gap (Eg) was studied. The optical band gap was found to be within the range of 3.50 to 4.0 eV.