Three-dimensional (3D) flower-like Ni(OH) 2 nanostructures have been successfully synthesized by a facile template-free solution process, in which F − and NH 3 were used as Ni 2+ coordination agents, and ammonia hydroxide solution was used as OH − supplier to accelerate the hydrolyzation of nickel complex species. The flower-like Ni(OH) 2 nanostructures consisted of many intercrossed nanoflakelets with a mixed crystal structure of α- and β-Ni(OH) 2 . The F − and NH 3 in reactive solutions played important roles in the nanostructure growth process. The Ni(II) concentration had great influence on the morphologies of Ni(OH) 2 nanostructures. The possible growth mechanisms including aggregation growth, surface growth and surface dissolution were also proposed. The magnetization measurement indicated that the flower-like Ni(OH) 2 nanostructures exhibited paramagnetic behavior above the critical temperature (10K) and ferromagnetic behavior below the critical temperature.