Crystallographic defects introduced in the YBa2Cu3O7−δ thin films during depositions were studied using X-ray diffraction method, and their influences on superconducting properties were also investigated by examining Tc variation of the as-grown and post-annealed thin films. It was found that there exist several kinds of defects in the as-grown thin films, such as oxygen deficiency, cation disordering, and others like lattice dislocation, stacking faults, etc.. These defects could be relaxed by heat treatment at temperatures ranging from 400 to 900 °C, resulting in Tc enhancement more or less. Quantitative understandings of the relations among annealing temperature, structural relaxation, and Tc improvement has also been reached. The kinetics of the relaxation of the defects in the YBCO films was studied further using a high-temperature X-ray diffractometer. Interesting phenomena have been observed at around 400 °C due to abnormal desorption behavior of oxygen and at higher temperatures relating to cation ordering.