In this paper, the structural, thermal and magnetic properties of Ni1-xMnxFe2O4 are presented. It is observed that high concentration of Mn2+ ions into NiFe2O4 tends to reduce the particle size. Calcination at 500 °C has resulted in the growth of Ni1-xMnxFe2O4 nanoparticles, but the calcination at 900 °C has led to the evaporation of the majorities of the polyvinyl alcohol. After calcination at 900 °C, crystallographically oriented NiMnFe2O4 nanoparticles are formed. These Ni1-xMnxFe2O4 nanoparticles show hysteresis behaviour upon magnetization. On the other hand, saturation magnetization (Ms) values decreases with increasing Mn content in ferrite due to the influence of Mn2+ ion in the sub lattice. © 2009.