The kinetic details of nucleation in supercooled liquid Na have been investigated by tracing the evolutions of dynamic and structural properties during the isothermal relaxation process. It is found that many different BCC clusters are formed with the disaggregations of icosahedron clusters during the alpha-relaxation regime. Those being smaller than the critical size are unstable due to their loose internal structures. The critical size of nucleus (about 100 atoms) obtained in present simulations does not deviate too much from that predicted by the classical nucleation theory. And the preferentially growing orientation of nucleus is close-packed (1 1 0) crystal face. (C) 2010 Elsevier B. V. All rights reserved.