Quantum well intermixing of In0.53Ga0.47As/In0.52Al0.48As multiquantum wells (MQWs) in an impurity free vacancy diffusion (IFVD) mechanism was investigated to observe the intermixing aspect and the effect of defects in MQWs with regard to the microstructural aspect using transmission electron microscopy. The MQWs were grown on a GaAs (100) substrate using compositionally graded buffer layers via molecular beam epitaxy, and the MQWs were annealed at 750 and 900 degrees C for 30 s via rapid thermal annealing for quantum well intermixing using IFVD method. In the fabricated MQWs, defects, such as stacking faults, twins and dislocations, were not generated at 750 degrees C. The diffusion of Ga in the well layer for the quantum well intermixing started from the top well layer, because the SiO2 layer that supplied vacancies for the quantum well intermixing was at the top of the sample. Additionally, in the same well layer, the intermixing did not show equality, because these vacancies were not supplied homogeneously. Especially, in the 900 degrees C annealed case, many dislocations were generated from the cladding layer. These dislocations contributed to new vacancy generation sites, thus the quantum well intermixing was accelerated.