This study focuses on the preparation and characterization of single phase NiO nanoparticles. At first, nickel-o-phthalate complexes as precursor were synthesized through semisolid phase reaction method and then NiO nanoparticles were obtained via a solid-state decomposition procedure of layered coordination nickle-o-phthalate complexes formulated as [Ni(pht)(H2O)2] and [Ni(pht)2]. In addition, the effects of calcination temperature and metal-to-ligand ratio on the particle size and morphology of NiO were investigated. Thermogravimetric analysis (TGA) was applied to determine the thermal behavior of complexes. The crystalline structure of products by X-ray diffractometer (XRD), morphology of particles by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were studied. The composition of as-prepared sample was studied by X-ray photoelectron spectroscopy (XPS) spectra. Analysis of FT-IR spectra confirmed the composition of products. The magnetic property was studied with vibrating sample magnetometer (VSM).