A novel PMMA/silicone semi-IPN, in which linear PMMA and silicone network were the guest and host polymers, respectively, was synthesized in this study. The silicone network formed by the self-condensation reaction among the pendent reactive methoxysilane groups on the polymethylphenylsiloxane(DC3074). Before curing, PMMA and DC3074 were miscible at any blended ratio, but phase separation occurred when the silicone network formed. The morphology of this semi-IPN was investigated by SEM and DSC, an inward-shift T-g for the pure components in the blends and a particular broad T-g were observed in the 50/50 blend. The phase structure for the blends wa;further investigated by the dynamic relaxation experiments of C-13-CP/MAS NMR spectroscopy. A multi-exponential relaxation rate of T-1 (H) at 50/50 ratio shows that this semi-IPN is not homogeneous at the interfacial on the length scale of 21 nm. The decrease of T-CH and the change of T-1p(H) for this semi-IPN, comparing to the pure materials, imply that intermolecular polarization transfer between PMMA molecules and silicone network is possible.