Optimal conditions for reducing threshold current are investigated for quantum structure (QS) semiconductor lasers. The filling factor of whole QSs to an active region is introduced. A general procedure is presented for obtaining the relation between the threshold current and filling factor from the relation of gain to injected carrier density of the individual QS. It is shown that there is a filling factor that minimizes the threshold current. From this value, the optimal areal coverage is shown to be about 0.1 for a quantum box and 0.8 for a quantum wire for an ordinary surface emitting laser with a single In0.53Ga0.47As/InP. The effects of various imperfections in actual QSs are also discussed. ©1995 American Institute of Physics.