Cobalt nanoparticles were loaded onto mesoporous zeolite Na-Y (Na-meso-Y) by melt infiltration and impregnation methods. As compared to impregnation, melt infiltration resulted in Co nanoparticles with a narrower size distribution. The Co/Na-meso-Y catalyst prepared by the melt infiltration exhibited higher C-10-C-20 (diesel fuel) selectivity. The hydrogenolysis of heavier hydrocarbons was confirmed to occur under Fischer-Tropsch reaction conditions. Our studies for the hydrogenolysis of n-hexadecane revealed that the catalyst by the melt infiltration was more active and selective for the formation of C-10-C-15 hydrocarbons. We propose that the narrower Co size distribution favors the selective hydrogenolysis and thus the C-10-C-20 selectivity in Fischer-Tropsch synthesis. The addition of Mn with a proper content could further improve the diesel fuel selectivity to 65% by suppressing the formations of CH4 and lighter hydrocarbons. The Mn-modified Co/Na-meso-Y catalyst was very stable and no deactivation was observed in 1000 h.