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The differential gain of a quantum‐well laser is studied theoretically with use of both a parabolic band model and a valence‐band‐mixing model. In the valence‐band‐mixing model, the gain profile is derived from the multiband effective mass theory (k∙p method) as well as the density matrix formalism. The peak gain including the band‐mixing effect is significantly reduced to 1.5–2 times when compared to the conventional parabolic band model. There is still a larger differential gain using the parabolic band model than using the band‐mixing model. The magnitudes of differential gains for these two models give the order of 10-16–10-15 cm2, which is in agreement with the experimental results. Besides, the quantum‐well thickness also influences the differential gain, which is enhanced by a thinner quantum‐well structure.