Symmetric electrochemical capacitors (ECs) have been fabricated using graphene nanosheets (GNs) modified with carbon nanotubes (CNTs) as electrode materials and LiClO4/propylene carbonate as organic electrolyte. An efficient homogenizing method is employed to insert CNTs into the GNs, forming three-dimensional CNT/GN framework. The electrochemical properties of as-prepared Li-ion ECs are well characterized by cyclic voltammetry, galvanostatic charge discharge cycling, and ac impedance spectroscopy. The CNT/GN-based EC delivers high specific capacitance of 107 F g(-1), low equivalent resistance of 33.0 Omega, and high Li+ diffusion coefficient of 1.09 x 10(-13) cm(2) s(-1), showing an obvious enhancement as compared to GN- and CNT-based ones. This improved performance can be attributed to the fact that the presence of CNTs facilitates not only more electro-active sites on the graphene surface but also better Li+ diffusion accessibility and electrolyte wetting in the carbon framework. The energy density of CNT/GN-based EC can reach as high as approximately 18 Wh kg(-1) at a power density of 3000W kg(-1), analyzed by the Ragone plot. Accordingly, the CNT/GN hybrid can be a promising electrode material for Li-ion ECs owing to its low cost, high performance, and easy fabrication. (C) 2015 Elsevier B.V. All rights reserved.