With the generalized simulated annealing algorithm (GSA), the structures and properties of Ni-n (n = 2-55) clusters have been studied. We find that besides the icosahedral-like structure, the structures of Ni clusters can be decahedral-like or fragments of the fee crystal. The clusters with n = 13, 38, 55 are found to be very stable. With modification of the effective coordination number model, the calculated ionization potentials of Ni clusters are in good agreement with the experimental data. The dynamical simulations show that the clusters with a closed-compact inner core have a high vibrational frequency mode, while the disordered clusters show some low vibrational frequency modes.