A solvothermal method was used to synthesize functionalized graphene, which exhibits an ultrahigh capacitance. This solvothermal method allows a fine control of the density of functionalities on graphene surface. The structure of resulting functionalized graphene is characterized by X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA), FTIR and Raman. Pseudocapacitance is provided by functionalities on graphene surface, such as carboxyl, carbonyl and hydroxyl. The significance of these functional also includes improving the wetting properties of electrode material, especially for supercapacitors using aqueous electrolyte. However, there is a penalty for functionalities since these oxygen-containing functional groups will disrupt the -conjugated system and lower the electrical conductivity. Therefore for functionalized graphene as supercapacitor, a tradeoff exists between the high psudeocapacitance and low conductivity, both are arising from the surface functionalities. Our systematic study shows a successful control of the density of functionalities, which is essential to achieve high performance of graphene-based supercapacitors. The capacitance of graphene is measured in a three electrode system using cyclic voltammetry (CV) and galvanostatic charging/discharging techniques. At a proper reduction condition, a high capacity of 276 F/g was achieved at a discharging current of 0.1 A/g in H 2SO4 solutions. The superior capacitive performance of functionalized graphene demonstrates the importance of surface property engineering, which will greatly promote the study and application of graphene-based supercapacitors.