The cluster variation method formalism in the irregular tetrahedron approximation has been applied to the determination of the anti-phase boundary (APB) energies of anti-phases generated by the slip of multiple dislocations on a single plane in BCC alloys. The formalism has been applied to BCC Fe–Al alloys and compared with experimental results in the same system. The results shows that in the short-range ordered (SRO) state, most of the resistance to dislocation motion is felt by the first moving dislocation, which characterises slip-plane softening. For the long-range ordered (LRO) state the calculation justifies the formation of superdislocation configuration as experimentally observed in the phases at room temperature deformation. The results have been discussed in connection with the experimentally observed deformation modes of BCC Fe–Al alloys.