Nickel nanomaterials with a wide range of morphologies and sizes, such as superfine nanoparticles, urchinlike chains, smooth chains, rings, and hexagonal Ni/Ni(OH)(2) heterogenous structure plates, are synthesized in a single reaction system by simply adjusting the reaction conditions. The morphology transformation mechanism is systematically investigated. Magnetic measurement of urchinlike chains, smooth chains, and rings shows that the saturation magnetization (M(s)) decreases with reduced sample size, and remanent magnetization (M(r)) decreases with increasing reaction temperature. Additionally, coercivity (H(c)) of urchinlike chains which is apparently larger than that of bulk nickel depends more on size than on shape anisotropy according to spherical chain reversal magnetization model. Enhanced microwave absorption of Ni/Ni(OH)(2) hexagonal plates compared with smooth chains and rings is due to the synergistic effect of magnetic loss and dielectric loss. Particularly, the urchinlike nickel chains exhibit a best absorption property in contrast with other as-synthesized samples and other reported nickel structures, which can be attributed to the geometrical effect, high initial permeability, point discharge effect, and multiple absorption. The prepared nickel nanomaterials can be applied as promising absorbing materials.