Ultrafine Ni(II)-bearing ferrites (UNFs) with 14-100% Ni(II)-substitution of x in Ni_xFe_3-xO₄ for Fe²⁺ in magnetite were synthesized by hydrolysis of Ni²⁺, Fe²⁺, and Fe³⁺ ions at 60°C,followed by aging of the co-precipitates at 60°C and eating to 300°C. The aged precipitates were the precursors of the UNFs, into which they were transformed by heating at 300°C The Ni(II)-substitution of the UNF was corroborated by a decrease in the lattice constant with an increase in the Ni(II) content of the product. The ferrite particles were nearly spherical and uniform. The average crystallite sizes ranged from 8 to 17 nm, which decreased with an increase in the Ni(II)ontent of the ferrite. In the average size range from 14 to 17 nm, the UNFs with various Ni (II)substitutions could be synthesized by changing the aging conditions. A transtion of the UNF (x= 1.0) from superparamagnetic to ferrimagnetic particles at room temperature occurred in the crystallite size range from 8 to 14 nm. The UNFs gave a room temperature saturation magnetization (M_s)much lower than the theoretical M_s.