The paper describes a multiscale experimental investigation of themechanical behavior of polymer nanocomposites with nanoscale fused silicainclusions with the objective to shed light into the effect of the hardnanoparticles on the quasistatic mechanical behavior of epoxy matrix and theimplications of the latter to the effective composite properties. The mainvariable in this study was the nanofiller volume fraction while the particlesize was either 15 nm or 100 nm. Localstrain measurements indicated strain field localization in the vicinity ofthe nanofillers at strains that macroscopically fall in the linearly elasticregime. The matrix strains were as high as three times the applied far fieldstrain at applied effective strains of ∼ 1%. At larger stresses the localstrain fields evolved to maxima that were considerably higher than theapplied strain, and they were affected by local particle density anddistribution. In composites with the largest particle volume fraction, 5vol.%, 100 nm fillers, neighboring particles located insmall proximities behaved as single large particles and often resulted inmatrix strain shielding thus decreasing the benefit of the largesurface-to-volume ratio and the associated efficiency in load transfer. Onthe other hand the 15 nm fillers resulted in more uniformlydistributed deformation compared to composites with 100 nmparticles.