In this work, for the first time, the rheological properties of nanofluids of lanthania nanoparticles (NPs) in ethylene glycol (EG) as functions of shear rate, volume fraction, and temperature were measured. The results showed that both EG and the nanofluids behave as non-Newtonian fluids at lower shear rates and transform to Newtonian fluids at higher shear rates. The values of viscosity for both EG and the nanofluids decrease with raising temperature. The viscosity of nanofluids increases with increasing the volume fraction of lanthania NPs. A maximum of 66.4% increase in viscosity of EG at 25 °C and shear rate of 60 s-1 was observed when 5% v/v of lanthania NPs was added. The nearly pure hexagonal lanthania NPs were successfully synthesized using hydrothermal method. The NPs were characterized using seven techniques including X-ray powder diffraction, transmission electron microscopy, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, far infrared spectroscopy, and ultraviolet-visible absorption spectroscopy.