This research was performed by first-principles calculations based on the density functional theory(DFT). The properties of graphene and h-BN nanosheets with different sizes were calculated. The quantum size effect and edge effect are studied systematically. In the calculations,we constructed the n × n(n = 6,7,8,…,24) supercells and took out one of the largest regular hexagon or approximate hexagonal nanosheets from the supercells. In order to increase the stability of the structure,each atom in the structu...