The lowest-energy structures and properties evolution with the size of Sc2nN2n nanocrystals (n = 3-54) are explored by first-principles calculations. The formation of cage-like Sc4N4 units is energetically favorable. In small-size nanocrystals, the Sc4N4 units prefer to arrange into the rectangular-like wire structures, whereas they arrange into the compact rectangular-like or cubic-like configurations in large-size nanocrystals. The Sc4N4 cage can be viewed as the embryo of ScN bulk crystal. With the obtained lowest-energy structures, we calculated some physical properties such as binding energies, HOMO-LUMO gaps, ionization potentials, and electron affinities. The evolution of these properties with the size was discussed. (C) 2012 Elsevier B.V. All rights reserved.