Graphene oxide was prepared through Hummers' method，then different reduced graphenes were prepared via reduction of graphene oxide with hydrazine hydrate for 1h、12h and 24h. X-ray photoelectron spectroscopy (XPS) was used for the characterization of graphene oxide and the reduced graphenes. The variation of the contents of carbon in carbon and oxygen functional groups and chemical compositions of graphene oxides were investigated through analysis the content of different carbon atoms in different reduced graphenes. The results showed that the reduction reaction was very fast in the first 1 h, the content of total oxygen bonded carbon atoms decreased from 83.6% to 22.1%, and then after the reduction rate became very slow. After 12h, the content of total oxygen bonded carbon atom is 19.56%, only 2.54% lower than that of 1h’s. At the same time, C-N was introduced in the graphene oxides; this increased the stereo-hindrance for hydrazine hydrate attacking the C-Oxygen groups, thus reduced the reduction rate. After reduction for 24h, there still exists 16.4% oxygen bonded carbon atoms and the total conversion ratio of graphene approaches 70%.