We present a spin density-functional theory (DFT) study for semiconducting ScN and Mn-substituted ScN. Their structural and magnetic properties have been investigated using the all electrons augmented spherical wave method (ASW) with a generalized gradient GGA functional for treating the effects of exchange and correlation. Band structure calculation show that ScN is semiconductor with a narrow indirect band gap Γ–X of 0.54 eV. The total-energy versus volume calculations show that ternary Sc0.75Mn0.25N nitride is more stable in face-centred tetragonal-rocksalt (fct-rocksalt) structure, found experimentally, than the perfect rocksalt one. Spin-polarized results, at theoretical equilibrium, indicate that the ground state of Sc0.75Mn0.25N is ferromagnetic with a high moment at Mn-atom (3.45μB), and zero moment on Sc and N ones. The magnetovolume effects of Mn-substitution in ScN lattice are discussed. The electronic structures analyzed from site/spin projected density of states and chemical bonding, for both the mononitride and the ternary alloy, are reported. A discussion of the structural and magnetic properties of Sc0.75Mn0.25N is given with a comparison to the ScN, in order to get insights of the Mn-substitution effects.Read less <