Abstract:On a new torsional fretting tester， the torsional fretting tests of polymethylmethacrylate （PMMA）flat against GCr15 steel ball (with diameter of 40 mm) were carried out with various angular displacement amplitudes from 1° to 30° and imposed a normal load of 100 N. Base on the analysis of the frictional kinetics of torsional fretting， the damage characteristics of PMMA also were discussed in detail. The results indicated that the behaviors of torsional fretting were strongly depended upon the angular displacement amplitudes and the number of cycles， which can be characterized by the curves of friction torque / angular displacement (T-θ).Tow kinds of T-θ curves (i.e. parallelogram and elliptical curves) were observed during the process of torsional fretting. Along the radial direction，the wear scar could be divided to three zones， i.e. the adhesive zone，micro-crack zone and severe damage zone， respectively. The damage of torsional fretting at lower angular displacement amplitude was very slight. And increased with the angular displacement amplitude， the spindle-like shape pits distributed along radiating patterns from contact center can be observed. The result of debris determined by gel permeation chromatography (GPC) showed that the molecular weight of PMMA decreased during the torsional fretting occurred .The wear mechanism performed as delamination and crack.ing.