The effect of thermal annealing on multiple stacked InAs/GaAs quantum dots (QDs) embedded in an Al0.25Ga0.75As/GaAs heterostructure has been investigated by transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The TEM images show that the size of the InAs/GaAs QDs decreased slightly with an increase in growth temperature and that InAs/GaAs QDs annealed at 800 °C contained many dislocations due to annealing. Temperature-dependent PL spectra showed that peaks corresponding to the interband transitions of the InAs QDs shifted slightly towards the low-energy side and that the full width at half maximum of the peak decreased with an increase in annealing temperature. The integrated PL intensity decreased with an increase in temperature due to nonradiative defects resulting from the existence of dislocations. These results can help improve understanding of the thermal annealing effect on the microstructural and the optical properties of multiply stacked InAs/GaAs QDs embedded in modulation-doped heterostructures. © 2003 American Institute of Physics.