For a real superlattice, fluctuations are always presented in the period lengths. The band structure of semiconductor superlattices under the effect of this periodic disorder has been investigated in this paper. The zone center and zone edge of the first subband of electrons and holes and the effective energy gap as functions of this fluctuation have been calculated. We discuss the dependence of the band offset on this fluctuation. Our calculated results can be used to explain some of the experimental observations.