The optical bistability (OB) of intersubband transitions in a semiconductor superlattice is stu -died by applying the general formalism of the Kronig-Penney model and density matrix method. The Maxwell-Bloch (MB) equations of two-subband model are derived, and the OB state equation for a ring cavity is obtained from the stationary solution of the MB equations. The conditions for realizing the OB and its characteristics are discussed.