Four dinuclear LnIIICuII complexes with Ln=Tb (1), Dy (2), Ho (3), and Er (4) were synthesized to investigate the relationship between their respective magnetic anisotropies and ligand-field geometries. These complexes were crystallographically isostructural, and a uni-axial ligand field was achieved by using three phenoxo oxygen groups. Complexes 1 and 2 displayed typical single-molecule magnet (SMM) behaviors, of which the out-of-phase susceptibilities were observed in the temperature range of 1.8–5.0 K (1) and 1.8–20.0 K (2). The Cole–Cole plots exhibited a semicircular shape with α parameters in the range of 0.08–0.18 (2.6–4.0 K) and 0.07–0.24 (3.5–7.0 K). The energy barriers Δ/kB were estimated from the Arrhenius plots to be 32.9(4) K for 1 and 26.0(5) K for 2. Complex 3 displayed a slow magnetic relaxation below 3.0 K, whereas complex 4 did not show any frequency-dependent behavior for both in-phase and out-of-phase susceptibilities, which indicates that easy-axis anisotropy was absent. The temperature dependence of the dc susceptibilities for the field-aligned samples of 1–3 revealed that the χMT value continuously increased as the temperature was lowered, which indicates the presence of low-lying Stark sublevels with the highest |Jz| values. In contrast, complex 4 displayed a smaller and temperature-independent χMT value, which also indicates that easy-axis anisotropy was absent. Simultaneous analyses were carried out for 1–3 to determine the magnetic anisotropy parameters on the basis of the Hamiltonian that considers B20, B40, and B60.