Early stage of phase separation by spinodal decomposition in mixtuer of poly (methyl methacryate) (PMMA) and poly (styrene-co-acrylonitrile) (PSAN) was investigated with time-resolved light sacttering. The results indicate that the spinodal decomposition at early stage follows Cahn's linearized theory. The Maximum growth rate Rmax of phase separation has been revealed for the first time by time resolved light scattering. The theoretical R(q) is in agreement with the experimental one. Therefore
the interficial free energy can not been ignored in expression of R(q).