A numerical Kramers–Kronig analysis is used to calculate the refractive‐index change &Dgr;ncaused by the injection/depletion of free carriers in various dopedn‐type GaAs. The analysis makes use of a carrier‐related absorption spectrum, which is established by using quantum theory as well as empirical relations and is confirmed by the experimental absorption data in the literature. We obtain the &Dgr;nvalues for various doping concentrationsNDand carrier concentrationsNat three wavelengths; &lgr;=1.06, 1.3, and 1.55 &mgr;m. The &Dgr;nvalue is positive for low‐Nregion, and increases gradually to its maximum which is around 10−4for &lgr;=1.06 &mgr;m. Thereafter, &Dgr;ndecreases rapidly to 0 asNincreases. The linear relation between &Dgr;nandN, as predicted by the Drude theory, only happens whenNis beyond a certain value. In this region, the &Dgr;nvalue may attain to −10−2atN=5×1018cm−3for &lgr;=1.55 &mgr;m. Because of this significant &Dgr;nvalue and its linear relation withN, the free‐carrier induced index‐change effect may find the applications in integrated optics and optical probing.