Er-substituted cobalt-ferrites CoFe2-xEr xO4 (0≤x≤0.04) were synthesized by sol-gel assisted auto-combustion method. The precursor powders were calcined at 673-873K for 4h, subsequently pressed into pellets and sintered at 1273K for 4h. X-ray diffraction (XRD) confirmed the presence of the spinel phase for all the compositions and, additional orthoferrite phase for higher compositions (x=0.03 and 0.04). The XRD spectra and the Transmission Electron Microscopy micrographs indicate that the nanocrystalline particulates of the Er-substituted cobalt ferrites have crystallite size of ∼120-200nm. The magnetization curves show an increase in saturation magnetization (MS) and coercivity (H C) for Er-substituted cobalt-ferrites at sub-ambient temperatures. MS for CoFe2O4, CoFe0.99Er 0.01O4, CoFe0.98Er0.02O4, and CoFe0.97Er0.03O4 peak at 89.7Am 2/kg, 89.3Am2/kg, 88.8Am2/kg, and 87.1Am 2/kg, respectively, at a sub-ambient temperature of ∼150K. H C substantially increases with decrease in temperature for all the compositions, while it peaks at x=0.01-0.02 at all temperatures. The combination of Er content - x∼0.02 and the temperature - ∼5K provides the maximum HC∼984kA/m. Er-substituted cobalt-ferrites have higher cubic anisotropy constant, K1, compared to pure cobalt-ferrite at ambient/sub-ambient temperatures. K1 gradually increases for all compositions in the temperature decreasing from 300 to 100K. While K1 peaks at ∼150K for pure cobalt-ferrite, it peaks at ∼50K for CoFe 0.99Er0.01O4, CoFe0.98Er 0.02O4, and CoFe0.96Er0.04O 4. The MS (∼88.7Am2/kg), at 5K, for Er substituted cobalt-ferrite is close to the highest values reported for Sm and Gd substituted cobalt-ferrites. The MS (∼83.5Am2/kg) at 300K for Er-substituted cobalt-ferrite is the highest among the lanthanide series element substituted cobalt-ferrites. The HC (at 5K) for Er substituted cobalt-ferrite is close to the highest values observed for La, Ce, Nd, Sm, and Gd substituted cobalt-ferrites.