The aim of this work is the synthesis, determination of composition, structure and antioxidant properties of nickel bis-ligand complex with 2-hydroxy-3-methoxybenzaldehyde 4-(2,4-dimethylphenyl)thiosemicarbazone (H2L). The experiment showed that ethanolic solution of Ni(OAc)2*4H2O reacts with H₂L taken in a 2:1 molar ratio forming a red-brown substance with composition Ni(H2L)(L)*C2H5OH. Singlecrystals were obtained as a result of its recrystallization from dimethyl sulfoxide. Its structure was established on the basis of X-ray analysis. It was found (the space group C 2/c, unit cell parameters [Å] : a = 38.146(4); b = 9.6601(8), c = 19.7446(12); a = 90o, b = 91,613(7)o, y = 90o; volume – 7272.87 Å3) that the coordination polyhedron of the investigated coordination compound has a distorted square planar structure. There are two thiosemicarbazone molecules that perform various functions in the internal sphere. The first molecule of the thiosemicarbazone behaves like a tridentate ligand and coordinates to the nickel central atom by deprotonated phenolic oxygen atom (d(C-O) = 1.304 Å), azomethine nitrogen atom, and sulfur atom forming two five- and six-membered metallacycles. This thiosemicarbazone is in the thiolate deprotonated form. This fact is proved by the distance d(C-S) = 1.738Å which corresponds to single bonds and the distance d(C-N)carbamide = 1.291Å, which corresponds to double delocalized bonds. In composition of monodentate thiosemicarbazone the distance d(C-S) equals 1.710 Å which corresponds to double bonds, and the distance d(C-Ncarbamide) corresponds to single bonds. Besides this in monodentate ligand the distance d(C-O)phenol is 1.358Å that corresponds to a phenol fragment with a protonated oxygen atom. Other interatomic distances and valent angles are standard for compounds of this class. The studied compound exhibits the antioxidant properties in the range of concentrations 10-5 - 10-7 mol/L with the half maximal inhibitory concentration IC50 = 0.5 mmol/L. The obtained data indicates that its antioxidant activity is 67 times higher than the characteristics of Trolox that is used in medicine as an antioxidant. The found properties of the studied complex are of interest to medicine from the point of view of expanding the arsenal of synthetic antioxidants.