High-pressure phase transitions and phonon-dispersion relations of BeO have been studied by an ab initio method. A phase transition is found from a wurtzite structure (B4) to a cubic sodium chloride structure (B1) at about 122 GPa, but without a similar phase transition from B4 to a cubic zinc blende structure (B3). At ambient conditions, the B1 phase is not stable because in the calculated phonon-dispersion relations several modes with negative frequencies in this phase have been detected. However, they change to positive quickly with increasing pressures. The B3 and B4 phases are both stable at various pressures although the B3 phase is energetic meta-stable. When considering the temperature effect, a B4 -> B3 phase transition can happen, but the transition pressure is higher than that of the B4 -> B1 phase transition at the same temperature. Therefore, the 83 phase cannot be obtained by simply increasing pressures at high temperatures. Although there is the same atomic coordination at the plane (111) in the B3 phase and the plane (0 0 0 1) in the B4 phase, their sound velocities are different. The sound velocities of the longitudinal wave along the plane (0 0 0 1) in the B4 structure are always greater than that of the plane (1 1 1) in the 83 structure, and this difference almost keeps a constant with pressures. We think this could be a universal result for all A(N)B(8-N) compounds. (C) 2010 Elsevier B.V. All rights reserved.