CdO and Al-doped CdO nano-crystalline thin films have been prepared on glass at 300 degrees C substrate temperature by spray pyrolysis. The films are highly crystalline with grain size (18-32 nm) and found to be cubic structure with lattice constant averaged to 0.46877 nm. Al-doping increased the optical transmission of the film substantially. Direct band gap energy of CdO is 2.49 eV which decreased with increasing Al-doping. The refractive index and dielectric constant varies with photon energy and concentration of Al as well. The conductivity of un-doped CdO film shows metallic behavior at lower temperature region. This behavior dies out completely with doping of Al and exhibits semiconducting behavior for whole measured temperature range. Un-doped and Al-doped CdO is an n-type semiconductor having carrier concentration is of the order of similar to 10(21) cm(-3), confirmed by Hall voltage and thermo-power measurements. (c) 2009 Elsevier B.V. All rights reserved.