A growth method for the physical vapor transport of high purity CdS single crystals in a capsule with component pressure is described. The stoichiometry of CdS crystals can be adjusted by the control of Cd temperature which is located in a reservoir. With purification technique of the starting materials. Single crystals can be grown at lower temperature than previously used. The photo luminescence spectra of CdS were measured at 11K using 457.9nm line from an Ar+ laser. It is found that the intensity of free exciton increases with times of purification while the intensity of bound exciton decreases. This is a new attempt to judge the purity of CdS single crystals by PL intensity ratio of free to bound exciton.