The single-crystalline nanobelts of monoclinic Ni(SO4)0.3(OH)1.4 have been synthesized on a large scale through a convenient, low-temperature hydrothermal method. The influences of reaction time, pH value, concentration of ammonia on the phase and shape evolution of nanobelts were systematically investigated. It was found that the initial layered Ni(OH)2 thin films as intermediates could be split and converted into the monoclinic Ni(SO4)0.3(OH)1.4 nanobelts through SO42- ions inserting process. The formation mechanism of nanobelts involved in the inserting process and the corresponding drive force have been investigated in detail by means of X-ray diffraction (XRD), and scanning electron microscopy (SEM), FT-IR spectra and pH value analyses. In addition to, we have also found that the ammonia molecules acted as both weak base and ligand agent was crucial to the controlling nucleation and inserting process in the formation process of nanobelts.