Postgrowth thermal annealing was applied to investigate the optical and structural properties of InxGa1−xN/GaN multiple quantum wells with high InN mole fraction. Thermal annealing at 900 °C results in a twentyfold increase of the integrated photoluminescence intensity. Photoluminescence emission is also improved from a broad band for the as-grown sample to two dominant peaks for the annealed sample. Cross-sectional transmission electron microscopy shows the existence of quantum dot-like islands in the wells for the as-grown sample but these islands are significantly reduced after thermal annealing at 900 °C.