In this article, microwave was used for producing nickel-manganese alloy from nickel (III) oxide-manganese dioxide mixture in vacuum, and the heating processes and reaction mechanisms were also explored. When the nickel (III) oxide-manganese dioxide mixture was 20.0 g, the mixture can be heated and decomposed as the microwave powers were higher than 542 W, and the minimum power value making the system warm up to granulate was lower than that of the single-nickel (III) oxide or the single-manganese dioxide. With 740 W microwave power, the mixture can be heated and decomposed to gather and form the large diameter spherical particles of smelting nickel-manganese alloy. The average sizes of the particle were mainly 0.25 mm and 0.32 mm in diameter, accounted for 40.5% or 26.1% (wt%), respectively. The sizes of the particles enlarged with the increase of microwave powers. The reaction mechanisms have been put forward, and the processes can be divided into five steps: the electric spark reaction, the first reaction, the secondary reaction, and granulation. The critical point of the first reaction and the secondary reaction was 560 degrees C and 1870 degrees C, respectively. It was found that the mid-heating rates were higher than the post-heating rates; moreover, the late stage was the cooling process. Besides, a three-point analysis leading to the phenomenon was proposed. When the microwave power was 800 W, the elemental content limit was 68.7% and 16.3% after the decomposition of 20.0 g nickel (III) oxide-manganese dioxide under vacuum and in air, respectively.