Early stage of spinodal decomposition kinetics of deuterated polycarbonate/ poly(methylmethacrylate) blend has been studied by the time resolved small angle neutron scattering measurements. A trapped miscible state was obtained through fast solution casting of film specimens. Time dependence of the structure factor, S(q,t), was measured after the specimen temperature was quickly increased to a final temperature which is above its glass transition temperature, Tg. Thus, the early stage of spinodal decomposition kinetics has been observed starting from the dimension (q-1) comparable to the single chain radius of gyration, Rg, for a binary polymer mixture. The results provide an unequivocal quantitative measure of the virtual structure factor, S(q,∞); the relationship of qm and qc through rate of growth, Cahn-plot analysis, and singularity in S(q,∞); the growth of fluctuation of qRg < 1 and intra-chain relaxation of qRg > 1; and also a clear proof of the Cahn-Hillard-Cook theory in the early stage of spinodal decomposition of a mean-field system.