Appling high temperature optical microscopy （HTOM）, the in-situ observations on the melting process of RBa2Cu3Oz（RBCO） oxide thin films were carried out. Combined the X-ray diffraction （XRD） pole figures with the scanning electron microscope （SEM）, the microstructure of RBCO thin films with different in-plane and out-of-plane orientations were studied. The relationship between the thermal stability of RBCO thin film and its microstructure was investigated. The conclusions of semi-coherent interface energy theory is in good agreement with the experimental results from HTOM, SEM and XRD. It is evident that the thermal stability of 0° in-plane YBCO grain surpasses the 45° one in the air. Additionally, it was found that a very small volume percentage of a-axis grain has an obvious influence on the decomposition behavior and thermal stability of SmBCO film.