Melamine used as a coupling agent was functionalized onto a carbon fiber (CF) surface in supercritical methanol to improve the interfacial properties of CF reinforced epoxy composites. Fourier transform infrared spectroscopy (FTIR), Raman spectra and X-ray photo electron spectroscopy (XPS) confirmed the successful grafting of melamine molecules onto the fiber surface. Scanning electron microscopy (SEM) images showed that melamine was grafted onto the CF surface uniformly and the surface roughness was enhanced obviously. Dynamic contact angle analysis (DCA) revealed the significant improvement in the surface energy and wettability. Compared with the untreated CF composites, the interfacial shear strength (IFSS) and inter-laminar shear strength (ILSS) of composites after melamine modification increased by 41.3% and 36.4%, respectively. The impact properties were also improved significantly. In addition, the reinforcing and toughening mechanisms were also discussed. Meanwhile, supercritical treatment did not decrease the single filament tensile strength obviously.