The interaction of cobalt atoms with silicon (1 1 1) surface has been investigated by means of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Besides the Co silicide islands, we have successfully distinguished two inequivalent Co-induced root 13 x root 13 reconstructions on Si(1 1 1) surface. Our high-resolution STM images provide some structural properties of the two different root 13 x root 13 derived phases. Both of the two phases seem to form islands with single domain. The new findings will help us to understand the early stage of Co silicide formations. (C) 2008 Elsevier B.V. All rights reserved.