The theoretical study on the differential gain of InxGa1−xAs/InGaAsP quantum‐well (QW) lasers lattice matched to GaAs is presented. These results were also compared with those of InxGa1−xAs/GaAs QW lasers. The differential gain of InGaAs/InGaAsP lasers continuously increases with the In composition. On the other hand, InGaAs/GaAs lasers show the decrease of the differential gain when the In composition exceeds about 0.22. The increase of the differential gain observed in InGaAs/InGaAsP lasers is mainly due to the increase of the subband energy spacing in the conduction band. The decrease of the differential gain observed in InGaAs/GaAs lasers is related to the decrease of the energy spacing in the conduction band and the decrease of the optical matrix element. ©1996 American Institute of Physics.