Abstract: In this paper, the fretting wear behaviors of ultra-high molecular weight polyethylene (UHMWPE) with different surface roughness were investigated using an Optimal SRV-IV oscillating friction and wear tester. It is found that the friction coefficient of UHMWPE decreased firstly and then increased while the specific wear rate monotonically increased as surface roughness increased under dry sliding. In order to identify the wear mechanisms of UHMWPE under different fretting conditions, the morphologies of the worn surfaces were analyzed by using optical microscope and scanning electron microscope. Under dry sliding conditions, obvious ploughing accompanied with local mild plastic deformation were observed on the worn region of a smooth surface UHMWPE. However, plastic deformation and adhesive wear became more severe with the increasing surface roughness, and the transfer films from UHMWPE were formed on the worn surface of the steel ball. Under water lubrication, friction coefficient and the specific wear rate decreased significantly and showed the similar tendency as that under dry sliding. Abrasive wear dominated the wear mechanism in the process of fretting under water lubrication.