Sliding wear was investigated for ZrO2 ceramics against M50 bearing steel under vacuum condition. The research was studied on the ball-disk machine under different loads and steady sliding velocity. The worn surfaces of tested samples were examined using optical microscope, scanning electron microscopy, atomic force microscopy and X-ray photoelectron spectroscopy. The results showed that the friction coefficient of ZrO2 was decided by the applied load, the critical load of ZrO2 was 10N applied load under vacuum condition, the specific wear rate of ZrO2 drop rapidly with exceeded applied critical loads. The ZrO2 ceramics materials have transferred on the M50 bearing steel surface under high vacuum condition. The dominant wear mechanisms of ZrO2 ceramics were mico-adhesion, tribochemical wear, brittle scraps and micro-furrow wear under vacuum condition.