The threshold currents for AlGaAs quantum well lasers are studied theoretically. The structure dependent gain coefficient is obtained by taking into account the electron distribution in valleys. Theoretical threshold current densities calculated using the gain coefficient agree well with reported experimental results for separate-confinement heterostructure lasers. A design procedure for low threshold current laser is elucidated. The lowest threshold currents are 570 and 53 μA per 1 μm stripe width for modified multiple quantum well lasers with 32 percent and 90 percent reflectivity facet mirrors, respectively.