We have investigated the structural and electronic properties of both bare and hydrogenated Si-m (m = 5,17, 29, 35,47) clusters using density functional theory. It has been found that the total binding energy decreases as the cluster size increases for both bare and hydrogenated silicon clusters. Moreover, optimization contracts the clusters: optimized geometry of bare clusters is quite different from those of hydrogenated clusters. The electronic structure calculations give the size-dependent band gap for the hydrogenated silicon clusters and the band gap increases as the cluster size decreases. (C) 2001 Elsevier Science B.V. All rights reserved.