In this paper, plate-like NiO nanoparticles were prepared by one-pot solid-state thermal decomposition of NICKEL (II) Schiff base complex as new precursor. First, the NICKEL (II) Schiff base precursor was prepared by solid-state grinding using NICKEL (II) nitrate hexahydrate, Ni (NO3) 2.6H2O, and the Schiff base ligand N, N'-bis- (salicylidene) benzene-1, 4-diamine) for 30 min without using any solvent, catalyst, template or surfactant. It was characterized by Fourier Transform Infrared spectroscopy (FT-IR) and elemental analysis (CHN). The resultant solid was subsequently annealed in the electrical furnace at 450 oC for 3 h in air atmosphere. Nanoparticles of NiO were produced and characterized by X-ray powder diffraction (XRD) at 2q degree 0-140o, FT-IR spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The XRD and FT-IR results showed that the product is pure and has good crystallinity with cubic structure because no characteristic peaks of impurity were observed, while the SEM and TEM results showed that the obtained product is tiny, aggregated with plate-like shape, narrow size distribution with an average size between 10-40 nm. Results show that the solid state thermal decomposition method is simple, environmentally friendly, safe and suitable for preparation of NiO nanoparticles. This method can also be used to synthesize nanoparticles of other metal OXIDEs.