The electronic and optical properties of boron arsenide (BAs) in the zinc-blende (ZB) and rock-salt (RS) phases have been studied by the density functional theory (DFT) method based on the generalized gradient approximation (GGA). Using the enthalpy–pressure data, the structural phase transition from ZB to RS is observed at 141GPa. Our calculated electronic properties show that ZB-BAs is a semiconductor, whereas RS-BAs is a semi-metal. Calculations of the dielectric function and absorption coefficient have been performed for the energy range 0–30eV. The dependence of pressure on band structure and optical spectra is also investigated. The results are compared with available theoretical and experimental data.