The paper describes a multiscale experimental investigation of the mechanical behavior of polymer nanocomposites with nanoscale fused silica inclusions with the objective to shed light into the effect of the hard nanoparticles on the quasistatic mechanical behavior of epoxy matrix and the implications of the latter to the effective composite properties. The main variable in this study was the nanofiller volume fraction while the particle size was either 15 nm or 100 nm. Local strain measurements indicated strain field localization in the vicinity of the nanofillers at strains that macroscopically fall in the linearly elastic regime. The matrix strains were as high as three times the applied far field strain at applied effective strains of ∼ 1%. At larger stresses the local strain fields evolved to maxima that were considerably higher than the applied strain, and they were affected by local particle density and distribution. In composites with the largest particle volume fraction, 5 vol.%, 100 nm fillers, neighboring particles located in small proximities behaved as single large particles and often resulted in matrix strain shielding thus decreasing the benefit of the large surface-to-volume ratio and the associated efficiency in load transfer. On the other hand the 15 nm fillers resulted in more uniformly distributed deformation compared to composites with 100 nm particles.