Chemical mechanical polishing slurry of copper usually contains more than four compositions, in which strong acids, alkalis or hazardous chemicals are normally employed. With these slurries, surface roughness less than 1 nm is difficult to obtain on the surface of copper after chemical mechanical polishing. It is a challenge to develop a kind of novel chemical mechanical polishing slurry for copper including three environment friendly compositions. In this study, a kind of novel chemical mechanical polishing slurry is developed consisting of silica, hydrogen peroxide and chitosan oligosaccharide, where all the three compositions are environment friendly. After chemical mechanical polishing, surface roughness R-a and peak-to-valley values are 0.444 and 5.468 nm respectively. Chemical mechanical polishing mechanism is elucidated by infrared and X-ray photoelectron spectra and electrochemical measurements. Firstly, Cu surface is oxidized by hydrogen peroxide, forming CuO and Cu(OH)(2). Then, CuO and Cu(OH)(2) are dissolved by H+ ions released by the ionization of chitosan oligosaccharide. Subsequently, Cu2+ ions are chelated by chitosan oligosaccharide molecules. Finally, the adsorbed layer is removed by silica nanospheres, generating ultra-smooth surface of copper. The findings propose a new route for fabrication devices of copper and other transition metals used in integrated circuits, graphene, transformers, batteries and electronics industries.