For two group-5B elements, arsenic and antimony, the total energies of the simple cubic (sc) and body-centered cubic (bcc) structures are calculated within the local density-functional formalism and the norm-conserving pseudopotential method. Calculated equations of states for the observed high-pressure phases, the sc phase of arsenic and the bcc phase of antimony, are in excellent agreement with the experimental ones. We find that the sc phase of arsenic is stable at pressures below 70.9 GPa and that of antimony is unstable over the range of pressure where the bcc phase of antimony has been observed.