The structural development during radical polymerization of a mixture of methyl methacrylate (MMA) and poly(ethylene-co-vinyl acetate) (EVA) was investigated by light scattering and optical microscopy. In the early stage of polymerization, the mixture of MMA/EVA was a single phase. As the polymerization of MMA proceeded, phase separation took place via spinodal decomposition (SD). This was supported by the characteristic change in light-scattering profile with reaction time. Analysis by the linearized and scaling theories showed that the reaction-induced phase separation behaviour was different from the familiar thermally induced SD during isothermal annealing after temperature jump. The scaling analysis showed that the final phase-separated structure may be formed by the interruption of a co-continuous percolation structure. Moreover, to investigate the effect of graft copolymer on SD, a poly(vinyl acetate) (PVAc) macromonomer with a methacrylate end-group was added to the MMA/EVA mixture to yield PMMA-PVAc graft copolymer during the polymerization. The incorporation of PVAc macromonomer increased the dominant wavenumber of the final frozen phase-separated structure (smaller periodic distance). The reasons seem to be that the concentration fluctuation growth is suppressed until the system reaches a deep quench depth at which the fluctuation should have a large wavenumber, and that the coarsening is retarded at the late stage by the graft copolymer formed in situ.