The first-principles calculations have been used to determine structures, stabilities, and electronic properties of the completely hydrogenated boron nitride sheets and nanoribbons. Calculations show that these hydrogenated boron nitride systems have favorable formation energies and they still maintain a hexagonal network structure during full geometry relaxation. The hydrogenated zigzag boron nitride nanoribbons with various widths generally have ferromagnetic metallicity in their ground states, while the hydrogenated armchair boron nitride nanoribbons behave as nonmagnetic semiconductors with the wide direct band gaps. Predicted electronic properties of these hydrogenated boron nitride sheets and nanoribbons show remarkable size and structural dependences. (C) 2010 Elsevier B. V. All rights reserved.