A new sulfur passivation technique for GaAs surface by using S2Cl2 has been developed as the substrate treatment method in heteroepitaxial growth of ZnSe films. A clean surface can be achieved by annealing the as-treated sample in vacuum at 600-degrees-C. ZnSe thin films grown by hot wall epitaxy on S2Cl2 passivated GaAs show improved crystalline quality compared with those treated by conventional method, as indicated by Raman scattering measurements.