Physical properties of ultra-high molecular weight polyethylene (UHMWPE) in retrieved artificial knee joints were investigated. Lamella thickness, crystallinity, and mechanical chain scission appeared to increase during service. Crystallinity-depth profile was related to the conformity of joint and the thickness of tibial component. For the joint of low conformity, stress, crystallinity, and lamellae alignment were highest at subsurface 1-2 ㎜ beneath the surface, where crack propagation and delamination eventually occurred. It was suggested that the thickness and conformity of UHMWPE part should be increased for better wear-resistant tibial component of knee joint prosthesis.