First-principles calculations based on density functional theory (DFT), GW and Bethe−Salpeter equations are performed on the bilayer of Hexagonal Boron Nitride (bilayer h-BN) to investigate the electronic structure and optical properties. Due to the quantum confinement effect and the less efficient electronic screening, the value of direct (indirect) band gap increases from a value of 4.45 eV (4.37 eV) within the density functional theory to a value of 6.74 eV (6.67 eV) within the GW many-body Green's function theory. The calculated optical absorption spectra are dominated by exciton states with binding energy about of 1.67 eV. The structure of bilayer h-BN is expected to have various remarkable properties and it is a potential elementary structure for future Boron Nitride -based nanoelectronics

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