PURPOSE: The aims of this study were to investigate mechanical properties and hydrothermal degradation behaviour of the cubic-containing translucent yttrium oxide stabilized tetragonal zirconia polycrystal (Y-TZP). MATERIALS AND METHODS: Four groups of Y-TZP (T, ST, XT, and P), containing different amount of cubic crystal, were examined. Specimens were aged by autoclaving at 122°C under 2 bar pressure for 8 h. Phase transformation was analyzed using X-ray diffraction (XRD) to measure phase transformation (t→m). KruskalWallis test was used to determine the difference. Surface hardness, biaxial flexural strength, and fracture toughness in values among the experimental groups and verified with Wilcoxon matched pairs test for hardness values and Mann Whitney U for flexural strength and fracture toughness. RESULTS: XRD analysis showed no monoclinic phase in XT and P after aging. Only Group T showed statistically significant decreases in hardness after aging. Hydrothermal aging showed a significant decrease in flexural strength and fracture toughness in group T and ST, while group XT and P showed no effect of aging on fractural strength and fracture toughness with P<.05. CONCLUSION Hydrothermal aging caused reduction in mechanical properties such as surface hardness, biaxial flexural strength, and fracture toughness of Y-TZP zirconia. However, cubic-containing zirconia (more than 30% by volume of cubic crystal) was assumed to have high resistance to hydrothermal degradation.Clinical significance: Cubic-containing zirconia could withstand the intraoral aging condition. It could be suggested to use as a material for fabrication of esthetic dental restoration.