Bi-layer all-ceramic dental crowns comprise an inner core made of alumina or zirconia and an outer porcelain shell. The veneer safe from damage is mainly depended on the properties of core materials due to the poor strength and toughness of porcelain. The fracture modes and crack transformation of zirconia/porcelain and alumina/porcelain bi-layer composites reveal the potential feasibilities of improving the fatigue life of all-ceramic restorations. The failure modes of bi-layer composites were confirmed under cyclic spherical fatigue loading by dynamic microcosmic observation. Crack modes and evolvement of bi-layered composites were explored inflecting with fatigue load, cycles, frequency and water environment. Microcosmic morphology of the samples was observed by high depth of field stereomicroscope to determine degree and modes of fatigue damage. The appearance of cone cracks of porcelain surface was almost consistent in both of zirconia and alumina bi-layer composites. However, the pattern and process of the damage are different under cycle fatigue load. Furthermore, process of fatigue damage of bi-layer composites were observed to failure rapidly under water environment in both of zirconia and alumina bi-layer composites.