The phase-separated structure and molecular mobility for core-shell type polymer particles composed of poly(butyl acrylate) (PBA) and poly(methyl methacrylate) (PMMA) have been examined by high-resolution solid-state C-13 n.m.r. spectroscopy. These types of polymer particles were prepared by the two-step emulsion polymerization of BA and MMA in the presence or absence of ally methacrylate (AMA) as a crosslinking agent. The analysis of the H-1-C-13 cross polarization process has revealed that molecular mobility of the main-chain of PEA is significantly restricted for sample BA2M prepared in the presence of 1.6 wt% AMA. H-1 spin-lattice relaxation time measurements in the rotating frame have also revealed that the PEA and PMMA phases are phase-separated in a scale of several nm for the polymer particles. Furthermore, the H-1 spin diffusion process, measured by the combined pulse sequence of the Goldman-Shen pulse sequence and the CP process, has been successfully analyzed in terms of the equation derived by the simple two-spin cross relaxation theory. The time constant T-SD for the H-1 spin diffusion between two polymers obtained through this analysis has been found to be a useful parameter to elucidate the phase-separated structure. On the basis of these results, it is concluded that the PBA/PMMA polymer particles have a rather incompletely phase-separated core-shell type structure but the addition of an appropriate amount of AMA improves such a structure by confining the distribution of PMMA domains around the surface of the polymer particles probably as a result of the preferential crosslinking in the PEA phase.