Abstract: Torsional fretting wear of 7075 aluminium alloy flats against GCr15 bearing steel ball was investigated under various relative humidities (RH=10%, 60%, and90%).The normal loads (20 N, 50N and 100N) and torsional angular displacement amplitude (from 0.1o to 15o) were selected. The fretting running behaviors, wear mechanism, oxidative behaviors and the effect of various relative humidity were analyzed in detail, in combination with the micro-examinations of scanning electron microscope (SEM), and X-ray Photoelectron Spectroscopy (XPS). The experimental results revealed that the relative humidity significantly influenced the friction behaviors, wear damage and RCFM of torsional fretting. The wear mechanisms were mainly delamination and abrasive wear in three fretting regimes under different relative humidities. The results showed that slight damage was presented at the contact edges in partial slip regime, strong plastic deformation accompanied with the damages occurred in mixed fretting regime, and a debris layer was observed on the wear scars in slip regime. The friction torques decreased with the increasing RH. Humid air was beneficial to the slipping in the contact interface and the oxidation debris played as a lubricant in the fretting progress.