High-resolution transmission electron microscopy (HRTEM) images showed that the lattice distortion of as-grown InGaP/InGaAlP multiple quantum wells (MQWs) appeared to be due to the small thickness of the alloyed layers, and that their distortion was relaxed owing to the high atomic mobility. High-angle annular dark-field scanning transmission electron microscopy images demonstrated that the chemical intermixing of Ga and Al atoms between the InAlP and InGaP alloy layers due to thermal annealing relaxed the stress of the InGaAlP layer. The atomic arrangements of the as-grown and annealed MQWs are described on the basis of the experimental results.