We present X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations of microstructure and strain relaxation in YBa2Cu3O7 (YBCO) thin films grown on SrTiO3 (STO). The observations confirm the existence of twins in the relaxed layers and their absence in the strained ones. Dislocations are also present at the interface. We discuss the respective roles of twins and dislocations in the relaxation process.