The mechanical properties of micron-sized composite polymer particles have been investigated by using a nanoindentation-based flat punch technique. The contact loaddisplacement relationship of particles has been established and the stress-strain relationship has been determined. An interesting size effect on mechanical properties of both polymer particles and metallized polymer particles has been found. The smaller the particle size is, the stiffer the particle is. Finite element analyses indicate that different mechanisms dominate the size effect of two types of particles.