Abstract Tetragonally distorted antiperovskite Mn 3 CuN 1− δ exhibits huge magnetostriction of up to 2000 ppm, which is reasonably ascribed to ferromagnetic shape memory effects. The uniqueness of Mn 3 CuN 1− δ as a ferromagnetic shape memory alloy is exemplified by its nitrogen deficiency. The easy axis for magnetization rotates from the longer a axis in Mn 3 CuN 0.97 to the shorter c axis in Mn 3 CuN 0.78 . The operating magnetic field for magnetostriction is much lower for Mn 3 CuN 0.78 than for Mn 3 CuN 0.97 , suggesting supermagnetostriction. These characteristic behaviors can be explained consistently in terms of low magnetocrystalline anisotropy, which is a striking difference from other ferromagnetic shape memory alloys such as Heusler Ni 2 MnGa.