This article presents the synthesis of two nanoparticles Ni(II) complexes, [Ni(FEB)(Bipy)(H2O)2]Cl.2H2O and [Ni(FEB)(Para)(H2O)2]Cl.5H2O (FEB = febuxostat, Bipy = 2,2′-bipyridine, Para = paracetamol). Elemental analysis, magnetic moment (μeff), molar conductivity, XRD, mass spectrometry (MS), UV-visible, FT-IR and TG-DTG studies were utilized to determine the chemical formulae and structures of synthesized complexes. Results from the FT-IR study suggest FEB, Bipy and Para chelated with Ni(II) ion through OS, NN, NO, respectively, as bidentate ligands. Magnetic moment and electronic spectra results indicate the complexes have an octahedral geometry. XRD pattern showed the polycrystalline structure of complexes with crystalline sizes 83.42 and 38.84 nm, confirming nanostructure of complexes. The thermal stability of complexes was investigated with TG-DTG analysis. Coats-Redfern and Horowitz-Metzger equations were employed to determine the kinetic parameters of thermal decomposition stages. Lastly, in vitro antioxidant activities of FEB and complexes were evaluated by DPPH assay. The antioxidant activities test exposed that complex [Ni(FEB)(Para)(H2O)2]Cl.5H2O has higher activities than complex [Ni(FEB)(Bipy)(H2O)2]Cl.2H2O.