The optical gains in AlGaInP/GaInP strained quantum wells using valence band structures by the second‐orderk⋅pmethod, with and without spin‐orbit split‐off (SO) band effect have been calculated. It was easy to overestimate the optical gain without considering the SO‐band effect, because a small spin‐orbit splitting energy for GaInP makes higher nonparabolicity of the valence bands. The SO‐band effect is particularly significant under tensile strain, since the SO band makes the effective mass very large due to the large interaction between the SO and light hole bands. ©1995 American Institute of Physics.