The growth of ultrathin silicide (similar to monolayer) has been studied by scanning tunneling microscopy. Ni atoms react with the Si(111)-7x7 surface to form clusters at room temperature. With increasing annealing temperature, they gradually coalesce and form monolayer islands surrounded by root 19x root 19 ring structures. The surface structure shows root 3x root 3-R30 degrees at 400 degrees C and 2x2 at 600 degrees C, The latter structure is a new finding for the Ni/Si system. It is suggested that the transformation of the surface structure on the monolayer island can be related to a change of the interfacial structure.