<p>Tetragonally distorted Fe-Co-V-N with a body-centred tetragonal (bct) structure has a high saturation magnetisation and a high magnetocrystalline anisotropy of ~ 1 MJ/m³ and is expected to be a new hard magnetic material. The effectiveness of the combined addition of the elements V and N for bct formation has been reported, but the mechanism remains unclear. In this study, the amounts of V and N in FeCo were varied systematically, and the optimal amounts were found to be approximately 20 at% V and 6 at% N. This optimal composition was also effective for the bct transformation, even in a 100-nm-thick film. The role of V in the bct transformation is assumed to be to lower the energy needed to change the lattice constant of FeCo and attract N. The role of N is also considered to extend the <i>c</i>-axis of the Fe-Co-V lattice, causing a bct transformation.</p>