Cobalt catalysts supported on activated carbon and on carbon nanotubes (CNTs) with different porosities were prepared by an incipient wetness impregnation method and characterized by a series of methods. The samples were reduced and then evaluated in a fixed-bed reactor for Fischer-Tropsch (FT) synthesis. The porosity of the carbon support greatly influenced the microstructure, the reducibility, the dispersion, and the FT performance of the cobalt catalysts. The carbon structure and the cobalt dispersion determined CO conversion. CNTs with larger pore sizes were more stable at high temperature in a H-2 atmosphere. The cobalt particle size impacted the CO turnover frequency (TOF) and the C5+ selectivity. Larger cobalt particles (up to 7 nm) resulted in higher TOF and C5+ selectivity; for cobalt particles larger than 7 nm, no such increase in these parameters was seen. The carbon support influenced the C5+ selectivity and the C5+ hydrocarbon distribution. Interestingly, the olefin/paraffin ratio of C-2 was lower than that of C-3 or C-4 and a positive relationship existed between the C-2-C-4 olefin/paraffin ratio and the C5+ selectivity.