The friction characteristics of SUS440C-Si3N4 lubricated by PTFE under cryogenic environment have not been researched well. The lubrication mechanisms in such friction condition need to be clarified to improve the fidelity of performance modeling for ball bearings used in liquid rocket engine turbopumps. This research experimentally investigated the friction coefficient of bearing materials via PTFE lubrication under cryogenic environment to clarify the friction characteristics of cryogenic turbopump ball bearings. These experiments revealed the dependance of the contact pressure on the friction coefficient. By several analyses, it was found that lower friction and better wear-resistance characteristics were caused by PTFE film that strongly adhered on metal fluoride formed on the surface of SUS440C due to moderate friction energy.