$MnxZn_{1-x}Fe_2O_4$ (x=0.69~0.74) powders synthesized by the thermal decomposition of organic acid salts. The obtained powders were uniform in composition and ultra-fine particle with about 400 nm. The amount of spinel phase of these powders was about 50% in X-ray diffraction patterns. The calcination of powder was carried out at $900^{circ}C$ for 2 hours in air. After the powders were calcined. the mean size of powder was about 500 nm and the amount of spinel phase was increased over about 65%. The maximum amount of spinel phase was about 75% in the specimen of X=0.72. The magnetic properties of calcined $Mn_{0.72}Zn_{0.28}Fe_2O_4$ powders were the best among the different among the different compositions.