By using high temperature optical microscopy (HTOM), the superheating phenomenon in YBa2Cu3Oz (Y123) oxide thin film was found by in-situ observation of its melting process. The origin of superheating is discussed in combination with the analysis of XRD pole figure and the unwettability of Ba-Cu-O melts. Besides, the relationship between the microstructure of YBCO thin film and its superheating degree was investigated. The film with lower fraction of interface defects is believed to have higher superheating degree. The results deduced from semi-coherent interface energy theory are in good agreement with the experimental results of HTOM, AFM and XRD.