The optical band gaps and crystal structure were investigated on niobium doped TiO2 (for atomic niobium concentrations ranging from 0.02 –0.06 at. % in the composite) prepared by high temperature diffusion method. The Nb:TiO2 films displayed an enhanced visible light absorption with a red shift of 18.2 nm of the optical absorption edge from 394 nm for pure TiO2 film to 412.2 nm for 0.04 at. % niobium concentration representing a band gap lowering of 0.181eV due to the donor–type behavior of niobium. As the niobium concentration increased, the enhancement in light absorption at the investigated concentration range goes through a maximum at 0.04 at. % of Nb5+ with minimum band gap of 3.017eV. Despite higher rutilization, at the doping temperature of 850oC used, crystal sizes (39–43 nm) obtained from X-ray diffraction spectra depicted a significant increase in surface areas which is attributed to retardation of anatase - rutile phase transformation caused by Nb:TiO2 matrix.