The viscoelastic properties of poly(methyl methacrylate) (PMMA)/grafted poly(butyl acrylate) (PBA) nanoparticle systems were investigated. The rubber particles consist of PBA core (60 nm in diameter) and grafted PMMA shell. The grafting degree, defined as the weight ratio of grafted PMMA to PBA particles, ranges from 0.8 to 1.5. Two series of samples, A series with 7.5 wt.% of PBA content and B series with 12 wt.% of PBA content, were used. The systems exhibited fast and slow relaxation process, the former reflecting the relaxation of the matrix PMMA chains and the latter being attributed to grafted PBA particles. For A series samples, time-temperature superposition (TTS) was held well over the frequency (omega) and temperature (T) ranges measured. However, for B series samples, TTS was not satisfied at low omega due to the particle-particle interaction of grafted PBA particles, although the samples obeyed TTS at high omega associated with the relaxation with entanglement of matrix PMMA. At high T and low omega region, the B series samples showed a sol-gel transition at elevating T and the critical gel behavior characterized with a power-law relationship, G' = GaEuro(3)/tan(n pi/2) ae omega(n), was observed. This behavior suggested formation of a self-similar, fractal structure of grafted PBA particles. The critical gel temperature (T (gel)) and the critical exponent (n) were determined for the B series samples. TEM observations revealed that as-prepared A and B samples had well-dispersed particles but the B samples after viscoelastic measurements had fragmented networks of the PBA particles, confirming that the sol-gel transition occurred for the PMMA/grafted PBA systems at elevating T.