In this study, hexagonal close-packed (hcp) and spherical-shaped Ni nanoparticles (Ni NPs) were prepared by NiO reduction through a thermogravimetric method. Different morphologies need to be used as catalysts in industries because of higher performances in operations. It showed that controlling the shape and morphologies is possible using the thermogravimetric method. The reduction process was performed using methane as the reducing agent at different temperatures under atmospheric pressure. Effect of operational parameters including reduction temperature (630-830 degrees C), NiO particle size (20-39.91 nm), and morphology (hexagonal and quasi-spherical) on the size and morphology of the final Ni NPs were investigated in the case of chemical reaction controlling step. Moreover, the raw materials and the as-prepared samples were characterized by the field emission scanning electron microscopy (FESEM), the X-ray diffraction (XRD), and the energy dispersive spectroscopy (EDS). The reduction rate was improved by increasing the reaction temperature and decreasing the size of NiO particles and correspondingly, the reduction duration of NiO NPs was shortened. Based on the XRD patterns, more than 92% of NiO powder was converted to Ni NPs and the average particle sizes varied almost insignificantly as the sizes of initial NiO particles and the reaction temperatures were varied. In addition, according to FESEM images, the hexagonal and quasi-spherical NiO particles were reduced to uniform Ni-hcp and spherical-shaped Ni NPs at 730 degrees C, respectively.