The microstructure variations upon post-growth thermal annealing at different temperatures of InGaN/GaN quantum well (QW) structures of different well widths are demonstrated. The high-resolution transmission electron microscopy images showed that with a small QW width (2 nm), thermal annealing tended to form quantum dots (QDs) of larger sizes (3-5 nm) to cover the designated QW layers. In this situation, strain might be well relaxed and quantum-confined Stark effect (QCSE) was reduced, resulting in a higher radiative efficiency. Also, with the formed QD structures, the expected stronger carrier localization (CL) effect might make a significant contribution to the enhanced radiative efficiency. On the other hand, with a large well width (4 nm), thermal annealing tended to make the QW boundaries clearer. In this situation, it seemed that either QCSE was enhanced or CL was reduced, resulting in degraded optical quality. (C) 2003 Elsevier Science B.V. All rights reserved.