The Cahn-Hilliard-Cook (CHC) theory with random phase approximation (RPA) for the time change in the structure factor in multicomponent polymer systems is used to describe the dynamics of the early stage spinodal decomposition. Explicit expressions on the time change in the structure factor for the block copolymer/homopolymer mixture have been obtained with the dynamics of microphase/macrophase separations of the mixture presented in detail.