The morphology and the size distribution of self-organized cobalt silicide nanostructures, grown on Si (111) substrates with controlled defects, have been investigated. An initial defect structure on the Si (111) surface is produced by quenching the substrate from just below the 7x7 reversible arrow '1x1' (disordered) phase transition temperature. This has produced predominantly the Si (111)-(7x7) reconstructed structure along with some disordered regions and defect lines on the substrate surface. The disordered regions contain randomly placed Si adatomring clusters or a lattice gas of ring clusters and small patches of root 7x root 7 R 19 degrees structure. These substrates have been preannealed for different durations before 0.5 monolayer Co deposition on them for forming CoSi2 by reactive deposition epitaxy. With increasing duration of substrate annealing and consequent reduction of defect density and surface roughness, a change of island morphology, a transition from bimodal to monomodal size distribution and an increase of average island size have been observed. Reduction of surface defects via substrate preannealing appears to lead to the growth of homogeneous nanostructures. (C) 2013 Elsevier B.V. All rights reserved.