Rare-earth ion (Er 3+ ) substituted ZnFe 2−x Er x O 4 nanoparticles at different Er concentrations (x = 0, 0.2, 0.4, 0.6, and 0.8) were synthesized by surfactant (polyethylene glycol) assisted co-precipitation method and studied for structural and magnetic properties of the synthesized nanoparticles. All the ZnFe 2−x Er x O 4 ferrite nanoparticles exhibited an average size of around 25 nm. It was observed that substituent Er 3+ predominantly occupy octahedral sublattice in spinel structure. The crystal lattice unit cell volume and magnetic disorder increased with increase of Er 3+ concentration. The magnetic parameters such as spin-glass or superparamagnetic blocking temperature (T B ), maximum magnetization (M max ), remnant magnetization (M r ) and coercive field (H c ) of the ZnFe 2−x Er x O 4 ferrite nanoparticles decreased with decreasing temperature (3–300 K). High and low magnetizations were observed for the nanoparticles at 3 K and 300 K, respectively. The observed magnetic properties were explained clearly by considering the substitution of Er 3+ ion in to the octahedral sub-lattice and diminishing the magnetic exchange interactions between Fe 3+ ions with [Ar] 3d 5 and Er 3+ ions with [Xe] 4f 11 5d 10 electronic configurations. The present studies confirm that Er 3+ substitution does not enhance magnetic properties of ZnFe 2 O 4 nanoferrites. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.