A simple methodology is proposed to quantitatively discuss the quality of the 3D dispersion of grafted silica nanoparticles (NPs) mixed with free chains in nanocomposites. 3D observations of NPs are obtained by transmission electron tomography (TEMT). The NPs are individualized from the tomograms by segmentation and watershed processing. Equivalent dispersions of spherical objects are then generated. By computing 3D Voronoi tessellation from the spheres, the local environment is investigated for each NP through geometrical measurements. Two types of populations of NP can be extracted: either densely packed or isolated. As the length of the free chains decreases, when the length of the free chains are close to the ones of the grafted chains, this second population becomes predominant and the quality of the dispersion gradually increases. The width of the local volume fraction distribution surrounding each NP can be considered as a quantitative parameter indicative of the homogeneity of the microstructure. It becomes narrower as the quality of the dispersion increases.