Alpha-Ni(OH)(2) nanobelts, nanowires, short nanowires, and beta-Ni(OH)(2) nanoplates have been successfully prepared in high yields and purities by a convenient hydrothermal method under mild conditions from very simple systems composed only of NaOH, NiSO(4), and water. It has been found that the ratio of NaOH to NiSO(4) not only affects the morphology of the Ni(OH)(2) nanostructures, but also determines whether the product is of the alpha- or beta-crystal phase. A notable finding is that porous NiO nanobelts were produced after exposure of the Ni(OH)(2) products to an electron beam for several minutes during transmission electron microscopy (TEM) observations. Another unusual feature is that rectangular nanoplates with many gaps were obtained. Furthermore, porous NiO nanobelts, nanowires, and nanoplates could also be obtained by annealing the as-prepared Ni(OH)(2) products. A sequence of dissolution, recrystallization, and oriented attachment-assisted self-assembly of nanowires into nanobelts is proposed as a plausible mechanistic interpretation for the formation of the observed structures. The method presented here possesses several advantages, including high yields, high purities, low cost, and environmental benignity. It might feasibly be scaled-up for industrial mass production.