Four complexes based on rare earth biradicals [Ln(hfac)₃(NITPhmbis)₂]₂ {Ln = La (1), Nd (2), Gd (3), Tb (4); hfac = hexafluoroacetylacetone; NITPhmbis = [1,3-bis(1′-oxyl-3′-oxido-4′,4′,5′,5′-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene]} were successfully synthesized under appropriate conditions. Single-crystal structures show that 1 is a mononuclear two spin complex and 2–4 are mononuclear three spin complexes, in which a NITPhmbis molecule acts as a bidentate ligand with one coordinated lanthanide ion. The four complexes are isomorphous, the coordination number around the lanthanide ion is eight, and the polyhedron is a 4,4-bicappped trigonal prism. For lanthanum(III) complex 1, the temperature dependence of the magnetic susceptibility indicates that weakly antiferromagnetically coupled radicals exist (J_rad–rad = –1.2 cm^–1). In the case of neodymium(III) complex 2, its magnetic behavior gives antiferromagnetic Nd^III–radical interactions, whereas for complexes 3 and 4, static magnetic properties show that the Gd^III and Tb^III ions interact ferromagnetically with the radical. The dynamic magnetic properties of 4 were also measured, and 4 was found to exhibit slow relaxation of the magnetization at low temperature.