Porous carbons are promising candidates for many industrial applications, such as gas/water purification, catalysis, gas-diffusion layer, and electrochemical electrode. Recently, mesoporous carbon, with mesopore as the major pore type, has been developed. Compared with the traditional active carbon, mesoporous carbon not only offers advantage in large-molecule adsorption, but more importantly, favors many other applications, such as catalyst support, electrical double-layer capacitors, electrochemical electrodes. In this study, we report an alternative way to synthesize mesoporous carbon with uniform and tunable pore size by combing the activation mechanism and the direct-synthesis method. The addition of chemical activation reagent, phosphoric acid or zinc chloride, into the silica/carbon precursor nanocomposites provides an additional parameter to tailor the pore size. When phosphoric acid was used as activation reagent, the obtained mesoporous carbons showed tunable pore size up to 15 nm, high surface area, large pore volume and narrow pore-size distribution; however, when zinc chloride was used as activation reagent, not only mesoporous carbons with controllable pore size were prepared by this method, porous silica/carbon nanocomposites were also obtained before silica removal.