We investigate the adhesion mechanism of aluminum spheres on a silicon substrate. Aluminum particles in the size range of 60-1500 nm were deposited onto a silicon substrate. It was found that aluminum particles underwent plastic deformation rather than elastic deformation because of van der Waals interactions. A finite element model developed recently is also used to analyze our results. Because the MP model was proposed to describe plastic deformation based on an analysis of metal microcontacts, our results prove the correctness of the MP model from its origin and basis of arguments.