Pure-phase cobalt nanocrystals with well-defined specific exposed facets were synthesized via controllable reduction of their CoO counterparts while retaining the same scale of particle size. Three different catalysts, i.e. hexagonal close-packed (hcp) Co pyramid with (10-11) and rodlike with (10-10) facets, as well as face-centred cubic (fcc) Co octahedron with (111) exposed were obtained and studied for Fischer-Tropsch synthesis (FTS) reaction. No noticeable changes of either the shape or the exposed facets were found under practical FTS reaction conditions. The hcp (10-11) facet exhibits the highest FTS activity and C5+ product selectivity with the lowest apparent activation energy and CH4 selectivity. Theoretical calculations of the energy barrier for CO dissociation and methanation of the reaction intermediate CHx rationalize the experimental results.