In this work, the mechanical properties of polymer matrix-ceramic fillers nanocomposites were investigated. A PCL (poly-caprolactone) matrix was reinforced with increasing amount of nano-sized silica particles in the range 1-2.5% by weight, and the resulting properties were determined as a function of reinforcement characteristics and volume fraction. In order to gain a deeper insight into the mechanical behaviour of such nanocomposites, a numerical model able to reproduce the peculiar composite features was set up. The study focussed on the effect of particles size and amount on the achieved increment in the overall stiffness. The computational approach revealed that a third phase, namely the interphase, has to be taken into account in the model in order to accurately reproduce the experimental results.