The lowest energy structures of Si-n clusters up to 21 atoms are optimized with a genetic algorithm (GA) and density functional theory (DFT) with generalized gradient approximation (GGA). The structural transition from prolate cage-based structures to near-spherical configurations is found at n = 17. Remarkable different melting behaviors of silicon clusters are obtained between those in prolate structures and near-spherical geometries. The structural transformation of near-spherical clusters is observed in the melting process. Two possible melting processes are proposed: near-spherical --> prolate --> subunits --> molten and near-spherical --> prolate --> molten oblate.