We study atomic and electronic structures of hexagonal boron nitride (h-BN) bilayers on the basis of the first-principles density-functional calculations. The interlayer distances between two atomic layers in h-BN bilayer and the band gaps are calculated under biaxial tensile strains. It is found that the interlayer distances are changed when the tensile strains are applied. It is also found that the band gap decreases as the tensile strain increases. The band gaps of h-BN bilayer are tunable by biaxial elongations.