Polymer nanocomposites (PNCs) have been widely investigated due to their significant performance in mechanical reinforcement, but the mechanism of mechanical reinforcement is still an open challenge. In this work, we focus on the modulus reinforcement mechanism of weakly attractive poly(methyl methacrylate)/silica nanocomposites in both rubbery and glassy states with variable nanoparticle (NP) size, the molecular weight of the polymer matrix, loading fraction, and interaction strength. We find that the modulus reinforcement in both rubbery and glassy states is strongly related to the interfacial layer. In the glassy regime, the intrinsic modulus increases monotonically with the ratio between the particle radius and the radius of gyration of the polymer matrix, which results from the synergistic effects from the extent of stretching of adsorbed chains and the number of contacts between polymer segments and NPs. After deducting the part of particle substitution contribution, the increase of the effective plateau modulus with the NP content is ascribed to the additional entanglements from the trapped chains in the loops of adsorbed chains. An exponential dependence of the average entanglement molecular weight on the available interaction sites is deduced from the effective medium concept, which gives a quantitative description of experimental results.