For quantum structure lasers, including quantum well, quantum wire and quantum box lasers, a simple method is presented to obtain the optimized structure of the active region for low threshold condition. A logarithmic description is used to describe the relationship between the material gain and the injected carrier density. Taking the InGaAs(P)/InP quantum-well laser and the InAs/GaAs self-assembled quantum box laser as examples, the optimal condition to obtain the lowest threshold current is provided, including the number of quantum wells, the area density of quantum boxes, and the cavity length.