Nickel substituted manganese ferrites, Mn_1-xNi _xFe₂O₄ (0.0 ≤ x ≤ 0.6), were fabricated by sol-gel method. The effects of sintering and substitution on their crystallographic and magnetic properties were studied. X-ray diffractometry of Mn_0.6Ni_0.4Fe₂O₄ ferrite sintered above 523 K indicated a spinel structure; particles increased in size with hotter sintering. The Mössbauer spectrum of this ferrite sintered at 523 K could be fitted as a single quadrupole doublet, indicative of a superparamagnetic phase. Sintering at 573 K led to spectrum fitted as the superposition of two Zeeman sextets and a single quadrupole doublet, indicating both ferrimagnetic and paramagnetic phase. Sintering at 673 K and at 773 K led to spectra fitted as two Zeeman sextets due to a ferrimagnetic phase. The saturation magnetization and the coercivity of Mn_0.6Ni_0.4Fe₂O₄ ferrite sintered at 773 K were 53.05 emu/g and 142.08 Oe. In Mn_1-xNi _xFe₂O₄(0.0 ≤ x ≤ 0.6) ferrites, sintering of any composition at 773 K led to a single spinel structure. Increased Ni substitution decreased the ferrites' lattice constants and increased their particle sizes. The Mössbauer spectra could be fitted as the superposition of two Zeeman sextets due to the tetrahedral and the octahedral sites of the Fe³⁺ ions. The variations of saturation magnetization and coercivity with changing Ni content could be explained using the changes of particle size.