Manganese nickel ferrite powder with general formula Mn0.4Ni0.6Fe2O4 was synthesized through oxalate precursor route. The effect of annealing temperature (400 - 1100oC) on the formation, crystalline size, morphology and magnetic propertieswas systematically studied. The resultant powders were investigated bythermal analyzer (TG-DTG-DSC),X-raydiffractometer (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). Based on thermal analysis results, the oxalate mixture decomposed thermally in multistep weight loss up to about 680oC. XRD indicated that Mn0.4Ni0.6Fe2O4 formed at much lower annealing temperature (=400oC) but contained á-Fe2O3 impurity. The hematite phase decreased by increasing the annealing temperature. The lattice parameters and crystalline size were increased with increasing annealing temperature. Single well crystalline ferrite was obtained at 1100oCwith crystallite size about 139 nm. The saturation magnetization of the ferrites powders continuously increased with the increase in annealing temperature. Maximum saturation magnetization (44.6 emu/g) was achieved for the formedMn0.4Ni0.6Fe2O4phase at annealing temperature 1100oC.