One of the phenolic acids is 4-hydroxybenzoic acid (HBA) which takes the form of a white crystalline solid with a molecular formula of C7H6O3, a melting point of 214.5 ℃ and a molecular weight of 138.12 g·mol-1. It soluble in polar organic solvents like acetone and alcohols, and slightly soluble in chloroform and water. The reactions between the metal ions and the HBA were carried out under specific conditions like (molar reaction was 2∶2 (ligand to metal), reaction temperature was 60 ℃, media was neutral (pH 7), and solvent was H2O ∶MeOH (1∶1). Under these conditions, the HBA was deprotonated to form (HOC6H4CO-2; L-). The ligand L- was coordinated to the metal ions forming the metal complexation. The reaction of 4-hydroxybenzoic acid (HOC6H4CO2H; HL) with the Ni(Ⅱ), Mn(Ⅱ) and Cu(Ⅱ) ions afford metal-complexes with gross formula of ［Ni2L2(NO3)2(H2O)4］, ［Mn2L2(NO3)2(H2O)4］ and ［Cu2L2(NO3)2(H2O)4］, respectively. These complexes were characterized by elemental analysis (CHN), magnetic susceptibility, UV-Vis spectra, infrared (IR), and X-ray powder diffraction (XRD) techniques. The complexes of HBA are insoluble in common solvents and hence molar conductance could not be measured, but this very insolubility indicates that the complexes are neutral. Data has demonstrated that the ligand (L-) was coordinated to the metal ion by bidentate bridging carboxylate group (COO-), with an octahedral geometry. Thus, HBA is expected to act as bidentate uninegative ions and the coordination number of the metal ions is six. XRD results showed that the complexes possess uniform and organized microstructures in the nanometer range with a main diameter in the range of 11～28 nm.