A series of cobalt embedded in nano-porous carbons (Co@C) with different particle sizes have been synthesized by one-step carbonization of metal-organic-framework (MOFs) ZIF-67. The particle size of Co@C can be determined by its precursor which is modified by the content of CTAB (cetyltrimethyl ammonium bromide). The as prepared catalysts were characterized by XRD, N-2 adsorption-desorption, SEM, TEM, TGA and H-2-TPR, showing not only high loading but also superior dispersion. The Co@C catalysts have similar morphology with their corresponding precursors, on which highly dispersed cobalt was achieved even at high Co loading. The particle size of Co@C catalyst decreases with the increase of CTAB content. When CO conversion is plotted as a function of CTAB content, a volcano-like curve is obtained with an optimum CTAB content at m((CTAB))/n((Co))= 8 (m stands for the weight of CTAB, n stands for the moles of cobalt). Among these catalysts, the 8CTAB-Co@C catalyst exhibited the smallest Co crystalline size, best Co dispersion and highest CO conversion, while the 16CTAB-Co@C catalyst showed the lowest activity due to the biggest cobalt crystalline size.