Zirconia, especially tetragonal zirconia polycrystal (TZP), has been widely used as a bioceramic due to its excellent mechanical properties as well as its tooth-like color, good biocompatibility, and chemical stability. TZP undergoes tetragonal to monoclinic phase transformation (T-M transformation). T-M transformation induced by stress enhances the mechanical properties, whereas that induced by moisture degrade the mechanical properties, which is referred to as low-temperature degradation (LTD). In addition, TZP is opaque due to birefringence of tetragonal phase. By the event that femoral zirconia implant was fractured due to LTD in 2001, LTD issue emerged seriously. As a result, the application of zirconia as a bioceramic was limited to dental field. Recently, on the demand of esthetics, appearance similar to natural teeth is required of dental restoration. Accordingly, cubic zirconia is introduced to TZP for the better translucency. Cubic zirconia is optically transparent and stable at low temperature, but it shows insufficient mechanical properties due to large grains. Therefore, when a cubic zirconia is introduced to TZP, it is important to improve the optical properties without decreasing the mechanical properties. First, by reducing the grain size of cubic zirconia polycrystal (CZP) with two-step sintering method, the mechanical properties are supplemented with maintaining the strengths of CZP and the applicability of CZP in dental field was confirmed. Secondly, by understanding LTD and by substantiating the mechanism of LTD, TZP, which has outstanding mechanical properties, was intended to be applied diversely as a bioceramic.