The purpose of this work was to calculate the electronic band structure of Cd1–xFexSe. Ab initio, calculations are performed in the Atomistix Toolkit program within the Densi-ty Functional Theory and Local Spin Density Approximation on Tight Tiger basis. We have used Hubbard U potential UFe = 2.42eV for 3d states for Fe ions. Supercells of 8 and 64 atoms were constructed. After the construction of Cd1–xFexSe (x=6.25%; 25%) supercells, atom relaxation and optimization of the crystal structure were carried out. Electronic band structure, and density of states were calculated, and total energy have been defined in antiferromagnetic and ferromagnetic phases. The band gap for the Cd1-xFexSe, x=0.06 in ferromagnetic phase is equal to Eg= 1.77 eV, in antiferromagnetic phase Eg=1.78 eV. For x=0.25 Eg= 1.92 eV. Antiferromagnetic phase considered more stable. Our calculations show that the band gap increases with the increases in Fe ion concentration.