Theoretical calculations of Al0.75Co0.25Sb diluted magnetic semiconductor (DMS) were investigated by using both GGA and TB_mBJ methods. First, the energy-volume optimization curves of AlAs and Al0.75Co0.25Sb compounds were plotted and the ferromagnetic (FM) phases were observed to be more energetically stable. The spinup states showed metallic properties while the spin-down states had a semiconductor nature. While these properties did not change, some differences were observed in the band gap values. Fermi energies were calculated as 0.298 and 0.462 R y and band gap values were obtained as 0.229 and 0.855 eV by using GGA and TB_mBJ methods, respectively. The values of the total magnetic moments did not change and they were obtained as 2.00 mu(B)/f.u. in both methods. Finally, the elastic properties of Al0.75Co0.25Sb compound were calculated. According to these results, Al0.75Co0.25Sb compound is a brittle half-metal ferromagnetic material and it is suitable for use in spintronic applications.