Al2O3 deposited via atomic layer deposition (ALD) has been used as an insulating and barrier film for thin-film transistors, organic electronics, and microelectromechanical systems. However, ALD Al2O3 films are easily degraded by hydrolysis under harsh hygrothermal conditions, owing to their poor environmental stability. In this study, the mechanical properties and water-vapor transmission rate (WVTR) of environmentally degraded Al2O3 films investigate by varying the temperature and relative humidity (RH). The hygrothermal environment lead to surface and pinhole-concentrated degradation based on aluminum hydroxide, which causes an increased WVTR and reduced elongation of the films in harsher environments. In particular, the elongation of the degraded Al2O3 films is reduced to 0.3%, which is one third of that of the as-deposited Al2O3, and its WVTR increases to the order of 10