Superparamgnetic Fe₃O₄ and RE:Fe₃O₄ (RE = Dy, Nd, La) nanoparticles with an average crystallite size in the range of 15–24 nm, were synthesized by co-precipitation method. The samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), UV–Vis spectroscopy, LCR bridge, and two-probe technique. X-ray diffraction patterns of all the investigated samples reveal the typical phase of magnetite structure, with a small contribution of orthoferrite (NdFeO₃) as a secondary phase in Nd:Fe₃O₄ sample. The saturation magnetization ( $M_{\text{s}}$) of the samples has values in the range from 41.8 to 52.3 emu/g, and decreases with RE ion doping depending on the ionic radius. Negligible values of the coercivity $H_{\text{c}}$ and remanence $M_{\text{r}}$, indicate the superparamagnetic nature of the investigated samples. The calculated values of indirect optical band gap of Fe₃O₄ and RE:Fe₃O₄ nanoparticles are in the range of 0.9–1.25 eV. The dielectric constant of the samples decreases, while their activation energy increases with the increasing of ionic radii of dopants.