The chemical additives in lapping slurry play a vital role in the efficient ultra- smooth machining of sap- phire wafer.In the investigation,the lapping experiment of sapphire wafer was performed by using a fixed abrasive pad,and the effects of the ethylene glycol concentration in lapping slurry on the material removal rate (MRR) and surface morphology of sapphire wafer were discussed.Then,the micro/nano- indentation technique and the photoe- lectron spectroscopy (XPS) were employed to analyze the physical and chemical properties of lapped wafer surface.The results indicate that (1) when the lapping slurry contains 5% ethylene glycol,the depth of surface- modified layer on the lapped wafer surface is about 3.90 nm,and is superior to that in the deionized water,which is 1.04 nm; and (2) higher ethylene glycol content can promote the surface- modified layer to form,which helps to improve the lapping efficiency and enhance the surface quality.