In this Letter, we employed an ion mobility technique to measure the size-resolved native work function of structure controlled nanoparticles in free flight. We found that the Fowler-Nordheim law is applicable to not only spheres but also aggregates. The measured work functions of spheres are size dependent, and consistent with classical image and Coulomb potentials explanations. On the other hand, the measured work functions of aggregates are independent of mobility size, but depend on the size of primary particles. In addition, the particle photoelectric yield of aggregates and spheres were also evaluated as function of incident photon energy. (C) 2011 Published by Elsevier B. V.