Ionic liquid based supercapacitors generally using nanoporous carbon as electrode materials hold promise for future energy storage devices with improved energy density, but their power performances are limited by the high viscosity and relatively large size of ionic liquid electrolytes. Understanding the relationship between the pore size of nanoporous carbon, the ionic liquid electrolyte diffusivity, and the energy/power density is critical for the development of ionic liquid based supercapacitors with high performance. Herein, we report the synthesis of single-layered mesoporous carbon sandwiched graphene nanosheets (sMC@G) with mesopore-dominant (82% similar to 89%) high surface area and tunable mesopore sizes (4.7, 6.8, 9.4, 10.6, and 13.9 nm). When using 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF(4)) as the electrolyte with a cation size of 0.76 nm, it is demonstrated that the ion diffusion coefficient increases a little when the mesopore size is not larger than 6.8 nm, and then jumps dramatically in the range 6.8-10.6 nm. When the pore size is enlarged to 13.9 nm, the ion diffusion coefficient increases slightly, approaching the bulk diffusion coefficient of the electrolyte. A size ratio of mesopore over electrolyte ion of 14 is recommended for fast ion/electrolyte transport and therefore improves the power density (14.7 kW kg(-1) at 20 A g(-1)) without compromising the energy density (130 Wh kg(-1) at 1 A g(-1)). The performance of sMC@G is superior to other porous carbon materials used in ionic liquid electrolyte supercapacitors.