The stabilities of different kinds of ordered structure in complex perovskites have been studied by the eight-point cluster variation method (CVM). The grand potentials of these structures have been calculated and compared with that of the disordered state. It has been proved that the -type ordered structure will be stable in the system when the nearest-neighbour interaction is much stronger than other longer-distance interactions. The - and -type ordered structures result from the next-nearest- and the third-neighbour interactions respectively. The grand potentials of the - and -type ordered structures reach minima at the composition x = 0.5, whereas the grand potential of one of the -type ordered structures reaches its minima at around x = 0.25 and x = 0.75. The formation of different kinds of ordered structure in real material systems has been discussed.