Hertzian cyclic fatigue properties of the alumina-glass dental ceramics were evaluated in exact in vitro environment at contact loads from 200 N to 1000 N and up to 10$^$6/ cycles to investigate the indentation damage and strength degradation. At 200 N, no strength degradation and crack generation was observed up to 10$^$6/ contact cycles. As load increased from 200 N to 1000 N, the drastic reduction in strength was found when the damage transition from ring to radial crack occurred. The extent of strength degradation was more pronounced in vitro environments probably due to chemical reaction of artificial saliva with glass phase through radial cracks introduced during large numbers of contacts.