Abstract: The fretting wear behavior of AZ91D magnesium alloy in slip regime was investigated on a Deltalab-Nene7 fretting wear test rig. The fretted surface morphologies were observed by means of optical microscope and scanning electron microscope. It was found that the fretting wear process of the Mg alloy could be divided into four stages, and the friction coefficient, fretting damage mechanisms, wear scar morphology, and wear debris detachment mechanism varied at different fretting-stages. The wear debris was mainly composed of metallic Mg and the mixtures of the oxides of magnesium, aluminium, and zinc. The hard wear debris led to abrasion to the Mg alloy.