Mixed ligand nickel(II) and ruthenium(II) complexes were synthesized from pyridoxal-N-allyl-thiosemicarbazone hydrochloride and triphenylphosphine. The structures of the complexes have been characterized by elemental analysis, IR, H-1 and P-31 NMR, conductivity, magnetic moment measurements and single-crystal X-ray diffraction technique. Based on X-ray crystallographic studies, a square-planar structure has been proposed for the Ni(II) complex, in which the thiosemicarbazone ligand acts as dianionic tridentate ONS ligand. In the case of the Ru(II) complex, the thiosemicarbazone is coordinated to metal atom as a monoanionic bidentate NS donor ligand in an octahedral geometry. Antioxidant activities of the ligand and its metal complexes were calculated as their trolox equivalent antioxidant capacities (TEAC) by CUPRAC method and DPPH assay. Both the ligand and its metal complexes were found to be antioxidant and are much more antioxidant at least 2.1 times than trolox, even, ligand is 3.5 times greater than that of trolox according to CUPRAC. A linear correlation (correlation coefficient R-2 = 0.9997) appeared between the obtained TEAC values by the two antioxidant assays.