In this paper We analyze the steady-state characteristics of bistable semiconductor laser diode (BILD). A simple model for optical output of BIDL is obtained using nonlinear rate quations for electron and photon densities. This model emphasizes the physical mechanisms and parameters responsible for the bistability, gives the state equation and explains the main features of BILD. Bistability with a very large hysteresis in P0-P4 characteristics is an outstanding feature of BILD.