Abstract: The aseptic loosening of prosthesis caused by wear debris has became the main factor affecting the life of artificial joint. The in vitro wear experiments of artificial knee joint were completed under 5 million wear cycles with CoCrMo-UHMWPE pair according to ISO 14243. The lubricants with wear debris were obtained by every 1 million wear cycles. Then, the UHMWPE wear debris was separated by acid treatment, and its quantity, size, shape and distribution characteristics were quantitatively characterized based on SEM micrographs. The results showed that in terms of shape, the proportion of granular debris could reach 62%, followed by the spherical debris which accounted for 12%, and the flakes, fibrillar, rods and other types of debris were less. In the process of wear experiments, the proportion of nanoscale debris was always above 30%, and with the increase of wear cycles, this proportion also gradually increased, up to above 80%. However, the total amount of wear debris increased first and then decreased. In addition, the cytotoxicity of different sizes and concentrations of UHMWPE debris on mouse fibroblasts was investigated. The results showed that the cytotoxicity of nano-scale debris was the strongest, and the cytotoxicity decreased with the size of debris increased from 5 μm to 25 μm. In the concentration range of 0.1~2.5 mg/mL, the greater the concentration of abrasive debris, the stronger the cytotoxicity was. Therefore, UHMWPE wear debris of artificial knee joints showed different characteristics in quantity, size, shape and their distributions with different in vitro wear cycles. The characteristics such as size and concentration of wear debris had important influence on cytotoxicity.