The linear viscoelastic behavior of "model" hybrid materials based on methyl methacrylate and methacryloxypropyl-grafted nanosilica was investigated. As unique features, the materials under study present an excellent dispersion of silica within the polymer matrix and are almost free of uncross-linked chains. In addition, very progressive changes in network architecture are available, resulting from changes in particle diameter, d, volume fraction of filler, Phi, number of methacryloyl units grafted per surface unit of silica particle, n, and nature of the grafting agent. The influence of these parameters on the characteristics of the mechanically active relaxations alpha and beta was examined. Emphasis was put on the storage modulus, E', on the loss modulus, E'', and on their dependence on filler volume fraction. E'' values were shown to simply account for the reduction of the mechanical energy lost within the material, in connection to the occurrence of polymer molecular motions. Analysis of E' variations as a function of Phi was based on the theoretical models available in the literature to account for the contribution of the spherical filler particles. In the glassy state, Kerner's and Christensen and Lo's models yielded comparable results. In the rubbery state, Guth and Gold's model was shown to prevail on Kerner's model.