Using ab initio crystal structure prediction, we have predicted several polytypic structures for B 2 O under high pressure. The structural parameters, total energies, electronic properties and hardness of the obtained structures have been studied using pseudopotential density functional theory within the local density approximation. We find that a monoclinic C2/m phase is the most energetically favorable structure; however, its X-ray diffraction pattern could not fit into the experimental data [J. Mater. Sci. Lett. 6 (1987) 683]. Instead, X-ray diffraction patterns of the energetically less stable tetragonal P4 2 mc and designed P-4m2 structures are in overall agreement with the experimental data. The simulated hardness of P4 2 mc and P-4m2 are 40.5 and 44.8GPa, respectively, in accordance with the experimental values of 33.5~40.5GPa.