Complexation properties of N-methyl-N-tolyl-2-(1H-benzimidazol-2-yl)pyridine-6-carboxamide (MeTolBIZA) with trivalent lanthanides were investigated by spectroscopic titration study and discussed based on structural character determined by X-ray crystallography. As a result, it is revealed that one or two MeTolBIZA molecules coordinate with lanthanide ions as tridentate ligand via two nitrogen atoms in 2-(1H-benzimidazol-2-yl)pyridine moiety and one oxygen atom in amide group. The stability constant of 1:2 metal-ligand complex decreases with increase of atomic number of lanthanide, and the 1:2 complex cannot be formed with heavier lanthanides. This result can be attributed to steric hindrance between ligand molecules.