A nonequilibrium phase transformation in stoichiometric Ni3Sn2during mechanical impact in a high‐energy ball mill was monitored by the measurements of high‐field magnetization, ac susceptibility, and x‐ray diffraction. X‐ray diffraction showed that the material transformed from the low‐temperature Ni3Sn2phase with Ni3Sn2‐type orthorhombic structure to the high‐temperature Ni3Sn2phase with partially filled‐up B8‐type structure after 40 h of milling. Quenching was used to confirm the phase transformation during ball milling. The magnetization of the ball‐milled high‐temperature phase (metastable) is much higher than that of the quenched high‐temperature phase, where both ball‐milled and quenched high‐temperature phases have higher magnetization than the low‐temperature phase. The Curie temperature of our metastable high‐temperature phase is quite close to that of the equilibrium phases (high‐ and low‐temperature phases). The result is discussed in terms of atomic disordering induced by mechanical milling.