Photoluminescence and stimulated emission (SE) in a wurtzite GaN bulk crystal and InxGa1-xN/GaN multiple quantum wells (MQW’s) are investigated under uniaxial stress applied perpendicularly to the c axis. The strain in GaN induces a decrease in the photoluminescence intensity of B excitons relative to A excitons due to an increase in the energy splitting between the two states. In InxGa1−xN/GaN MQW’s, SE and optical gain in the localized states are observed at 6 K under low excitation power below the Mott density of excitons. The uniaxial stress induces a low-energy shift of the gain peak and a decrease in the threshold carrier density of SE. In the excitation power above the Mott density, the SE arises from an electron-hole plasma recombination. The gain value at 0.43 GPa is 1.34 times as large as that without stress at 6 K, which is comparable with a theoretical estimation. The observed effects of strain are ascribed to a decrease in the density of states at the valance-band maximum. © 2003 The American Physical Society.