Abstract: The fretting and damage behavior of 2091 aluminum lithium alloy under radial and composite (tangential and radial) fretting against SAE-52100 steel at various loads and inclined angles were investigated in a ball-on-plate configuration. The cross-sectional morphologies of the fretted tracks of the Al-Li alloy at different stages were observed on an optical microscope, while the morphologies of the worn surfaces were observed on a scanning electron microscope, so as to determine the factors controlling the initiation of fretting white layer (i.e., the tribologically transformated structure, coded as TTS). Besides, the evolution of the TTS at different fretting stages was analyzed in detail. The results showed that no TTS was observed under radial fretting condition. The tangential stress and displacement of the contact surface were the factors controlling the initiation of TTS, which supported that the formation of TTS was characterized by plastic deformation. The TTS played a very important role in determining the fretting behavior of the Al-Li alloy/steel pair and exhibited different features in different stages of composite fretting.