Manganese nickel ferrite (Mn_0.2Ni_0.8Fe₂O₄) powder was synthesized through oxalate precursor route. The effect of annealing temperature (400℃ - 1100℃) on the formation, crystalline size, morphology and magnetic properties was systematically studied. The resultant powders were investigated by thermal analyzer (TG-DTG-DSC), X-ray diffractometer (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). Based on thermal analysis results, the oxalate mixture decomposed thermally in multisteps weight loss up to about 680℃. XRD indicated that Mn_0.2Ni_0.8Fe₂O₄ formed at much lower annealing temperature (≤400℃) but contained α-Fe₂O₃ impurity. The hematite phase decreased by increasing the annealing temperature. The lattice parameters were increased with increasing annealing temperature up to 1000℃. The average crystalline size increased by increasing the annealing temperature. Single well crystalline ferrite was obtained at 800℃with crystallite size about 109 nm. The saturation magnetization of the ferrites powders continuously increased with the increase in annealing temperature. Maximum saturation magnetization 48.2 emu/g was achieved for the formed Mn_0.2Ni_0.8Fe₂O₄ phase at annealing temperature 1100℃.