The disclosure provides a method of forming a ferromagnetic material, including: forming a magnetic element layer on a semiconductor layer formed on an inhibition layer; and forming a ferromagnetic layer of a Heusler alloy layer on the inhibition layer by heat treatment to induce the semiconductor layer and the magnetic element layer to react with each other, and a transistor, and a method of manufacturing the same. The inhibition layer for inhibiting a reaction of the semiconductor layer and the magnetic element layer restricts a semiconductor to be supplied for a reaction of the semiconductor and the magnetic element. Therefore, it is possible to form a ferromagnetic material having a high composition ratio of a magnetic element.