A continuing challenge of polymer nanocomposites (PNCs) is to establish the relationship between the interfacial layer chain packing and dynamics. In this study, we report the spatial distribution and dynamics of polystyrene-grafted silica (SiO2-PS) particle filled PS and poly(methyl methacrylate) (PMMA) nanocomposites. The results reveal that tunable specific spatial distribution of grafted nanoparticles can be obtained through the control of grafted chain polymerization degree (N) and grafting density (σ). The bulklike α-relaxation dynamics of PS matrix is unaffected by the silica NP loadings, while it is shown to slow down the α-relaxation dynamics in PMMA nanocomposites. More interestingly, we probe the enhanced mobility of interfacial layer (α′-relaxation) in PNCs filled with grafted NPs, which is further corroborated by the accelerated Maxwell-Wagner-Sillars (MWS) polarization process. Moreover, it is unexpected that the α′-relaxation time increases with increasing temperature in the vicinity of glass transition temperature (Tg) of polymer matrix. Such an anomalous temperature dependence behavior can be attributed to the influence exerted by the slower α-relaxation dynamics. Considering the phenomena presented above, we propose the three-layer model to explain the observed behavior of grafted silica NP filled nanocomposites. These findings provide new insight into the mechanisms responsible for mechanical reinforcement and therefore guide the design of PNCs with tunable macroscopic properties.