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The variation of the threshold current on the cavity length in strained‐layer InGaAs/GaAs quantum well lasers was studied both theoretically and experimentally. The radiative recombination rates were calculated, while the nonradiative recombination rates were described phenomenologically. Broad‐area lasers of various cavity lengths were fabricated on the same wafer for use in the experiments. The valence subband structures were calculated from the Kohn–Luttinger Hamiltonian plus the strain Hamiltonian with appropriate boundary conditions. When lasers were forward biased, the quasi‐Fermi levels were determined by the charge neutrality. From the dielectric function in the self‐consistent‐field method including the band‐gap shrinkage effect, the gain and the spontaneous emission rate spectra were obtained. At the threshold of lasing, the current was a sum of the radiative and nonradiative components. In the nonradiative component, we consider two mechanisms: the Auger and the interface recombinations. We found that (1) each subband structure possesses a cutoff in k space; (2) the dominant polarization of the emitted light from lasers under investigation is in TE mode; (3) for long cavity lengths, currents originating from the radiative and interface recombinations are dominant, while for short cavity lengths, current originating from the Auger process is dominant; and (4) as the cavity length decreases, the threshold current first decreases and then drastically increases. Therefore there is an optimum cavity length. Theoretical and experimental results were compared and presented.