In this work, the peak modal gain and the radiative and Nonradiative
current densities for Single Quantum Well (SQW) GaAs/AlGaAs laser are studied theoretically with varying the well widths Lz=(200, 150, 100, and 75) Ǻ, at a bandgap discontinuity ΔEc of 0.1 eV, it was found that the highest value of the peak modal gain gmax=400 cm-1 is achieved at Lz=75 Ǻ, and the lower value to achieve transparency Ntr=0.5×1018 cm-3 at Lz=200 Ǻ .The optimum value for QW width to achieve the lower threshold current density Jth=481.5 A/cm2 at the same injected carrier density is Lz=100 Ǻ.