Metallic cobalt clusters are synthesized inside the cages of NaY zeolite by a method including four steps, i.e.: (1) ion exchange, (2) precipitation of exchanged cobalt cations, (3) calcination and (4) reduction of the calcined oxide nano-particles inside the zeolite. As compared with that prepared by the ion exchange followed directly by calcination and reduction and that by the conventional impregnation method, the sample by this four-step method exhibits higher CO conversion and higher selectivity to n-C-10-C-20 paraffins in Fischer-Tropsch (FT) synthesis. The small metallic cobalt clusters inside the supercages of NaY zeolite probably account for the high catalytic performances. (C) 2003 Elsevier Science B.V. All rights reserved.