Vacuum heat treatments were performed at different temperatures for the Zr-Sn-Nb alloys which were already oxidized for a certain time. Results show that the oxide film dissolution into the matrix occurs during heat treatment, and the oxygen diffusion from zirconia to nearby zirconium matrix is enhanced. Diffusion kinetics of oxygen is discussed, and the diffusion coefficient is calculated for the specific alloy. The diffusion may be attributed to the existence of oxygen content gradient and the high oxygen solubility in the matrix. ZrO is observed through microscopic chemical analysis, and the thickness of the metastable layer increases after the heat treatment. The thickness of oxygen-dissolved zirconium matrix (Zr(O)) layer also largely widens. It is speculated that in the actual aqueous corrosion procedure, there should be co-existence of oxidation and oxide film dissolution into the matrix. When the oxidation rate is restricted, the oxide film dissolution becomes obvious and facilitates the growth of metastable layer.