Abstract: The periodical change of the friction coefficient in the sliding of PEEK,LDPE, and carbon fiber reinforced PEEK against SAE52100 steel was investigated with a pin on disc test rig. It has been found that the friction coefficients show a periodical reduction and the wear of LDPE shows a periodical platform as well. This is because the rubbing surfaces of the polymers transfer from glass like state to viscous flow state and softening and flowing of the polymers are resulted from the frictional thermal effect.