The geometrical and electronic structures of the germanium clusters with up to 25 atoms are studied by using density-functional theory with the generalized gradient approximation. The Gen clusters follow a prolate growth pattern with n>13. For medium-sized clusters, we find two kinds of competing structures, stacked layered structures and compact structures. The stacked layered structures with capped tetrahedron Ge9 cluster are more stable than compact structures and other stacked structures. The size dependence of cluster binding energies, highest-occupied and lowest-unoccupied molecular orbital gap, and ionization potentials are discussed and compared with experiments.