The method of the plane-wave expansion of envelope functions,as applied to calculating the subband structures in superlattices,is extended to the 8-band k·p Hamiltonian,in which both Γc1 and Γv15 bands are included.In this method,we can easily take into account of the effects of the nonparabolicity,the discontinuity of the effective masses,the spin-orbit splitting and the inversion asymmetry on subbands.Using this method,we have computed the electronic structure and the optical transition probability of two superlattice systems,in which the in-plane polarized interconduction subband transitions were observed.Our results show that the probability of such transitions is very small.It is thus expected that there might be other machanism to be responsible for the experimental results.