Ten new 3D lanthanide-based compounds, {[Ln4(CPOA)6(H2O)4]·H2O}n (Ln = Eu (1), Gd (2), Tb (3), Dy (4), Ho (5)) and [Ln(CPOA)(C2O4)0.5(H2O)]n (Ln = Eu (1a), Gd (2a), Tb (3a), Dy (4a), Ho (5a)) (CPOA2– = 4-carbomethoxybenzoate), have been synthesized by a hydrothermal method under different temperatures. The compounds 1–5 were obtained at 140 °C. When the temperature went up to 180 °C, the ligands partly decomposed to C2O42–, then came into being compounds 1a–5a. The two series of compounds displayed the differences not only in structure but also in thermal stability and luminescent and magnetic properties. Compounds 1–5 crystallize in the triclinic system, space group P1̅, whereas compounds 1a–5a belong to the monoclinic system with space group P21/c. Thermal analyses were carried out on 1–5 and 1a–3a. Solid luminescence studies of compounds 1 and 1a, 3 and 3a, and 4 and 4a show the characteristic bands of Eu3+, Tb3+, and Dy3+, respectively. Magnetic properties were investigated on 2–4 and 2a–4a. The out-of-phase signals (χM″) of 4 exhibit a strong frequency dependence, suggesting the existence of slow magnetic relaxation behavior in 4.