Abstract: Five graphene oxide (GO) with different oxidation degrees were prepared by the modified Hummers method. The composition and structure evolution of prepared GO were studied by elemental analysis, X-ray photo-electron spectroscopy and FTIR spectroscopy. The disperse states of different GO in the epoxy hardener (isophorone diamine, IPDA) were observed by an optical microscope. The GO sample which showed the best dispersibility was used to modify carbon fiber/epoxy(CF/EP) composites. The results indicate that by increasing the quantity of oxidant and reaction time, the oxidation degrees of GO increase accordingly. At a low oxidation degree, the functional groups on GO surfaces mainly contain carboxyl, carbonyl and phenolic groups. As the oxidation degree increases, the ether, epoxy and aliphatic hydroxyl groups gradually become dominant on GO surfaces. The disperse state of GO in IPDA is closely associated with the oxidation degree of GO. Both GO with the low and high oxidation degrees show clear aggregation. In addition, the GO surfaces can be chemically functionalized by IPDA during the dispersing process. Compared with the control sample with the unmodified EP matrix, the CF-GO/EP composites show about 14%, 17% and 14% increases in flexural strength, interlaminar shear strength and mode Ⅱ critical strain energy release rate (G_ⅡC), respectively, when the GO(0.2wt%) is added which has the best dispersibility is incorporated in the EP matrix.