Chemical Bath Deposition (CBD) technique was used for preparation of CdS, and Al doped CdS thin films. Al doped CdS films were annealed at different temperatures in air. CdS, as- deposited Al doped CdS and annealed Al doped CdS films were characterized using XRD, Scanning Electron Microscopy (SEM) and UV-Visible spectrophotometer. XRD study revealed the hexagonal phase of CdS and Al doped CdS material in the thin films. The XRD study shows that the hexagonal structure of CdS is not much affected with respect to Al doping. Al doped films annealed above 473 K crystalline phase of CdS transform into polycrystalline. The variations of lattice parameters and grain size with annealing temperature were investigated from XRD data. The % transmittance and optical band gap were investigated from optical data. The optical band gap of Al doped CdS films annealed at 473 K and 573 K is increased as compare to CdS (2.42 eV).