Electronic and structural properties of antiphase boundaries in group III-V semi-conductor compounds have been receiving increased attention due to the potential to integration of optically-active III-V heterostructures on silicon or germanium substrates. The formation energies of {110}, {111}, {112}, and {113} antiphase boundaries in GaAs and GaP were studied theoretically using a full-potential linearized augmented plane-wave density functional approach. Results of the study reveal that the stoichiometric {110} boundaries are the most energetically favorable in both compounds. The specific formation energy gamma of the remaining antiphase boundaries increases in the order of gamma({113}) approximate to gamma({112}) < gamma({111}), which suggests {113} and {112} as possible planes for faceting and annihilation of antiphase boundaries in GaAs and GaP.