In this study, nanocrystalline Co–Ni–Mg ferrite powders with composition Co_(0.5)Ni_(0.5-x)Mg_xFe_2O_4 are successfully synthesized by the co-precipitation method. A systematic investigation on the structural, morphological and magnetic properties of un-doped and Mg-doped Co–Ni ferrite nanoparticles is carried out. The prepared samples are characterized using x-ray diffraction(XRD) analysis, Fourier transform infrared spectroscopy(FTIR), field emission scanning electron microscopy(FESEM), and vibrating sample magnetometry(VSM). The XRD analyses of the synthesized samples confirm the formation of single-phase cubic spinel structures with crystallite sizes in a range of ~ 32 nm to ~ 36 nm. The lattice constant increases with increasing Mg content. FESEM images show that the synthesized samples are homogeneous with a uniformly distributed grain. The results of IR spectroscopy analysis indicate the formation of functional groups of spinel ferrite in the co-precipitation process. By increasing Mg2+substitution, room temperature magnetic measurement shows that maximum magnetization and coercivity increase from ~ 57.35 emu/g to~ 61.49 emu/g and ~ 603.26 Oe to~ 684.11 Oe(1 Oe = 79.5775 A·m-1), respectively. The higher values of magnetization Ms and Mr suggest that the optimum composition is Co_(0.5)N_(i0.4)Mg_(0.1)Fe_2O_4 that can be applied to high-density recording media and microwave devices. In this study, nanocrystalline Co-Ni-Mg ferrite powders with composition Co_ (0.5) Ni_ (0.5-x) Mg_xFe_2O_4 were synthesized by the co-precipitation method. A systematic investigation on the structural, morphological and magnetic properties of un- The prepared samples are characterized using x-ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), and vibrating sample magnetometry VSM). The XRD analyzes of the synthesized samples confirm the formation of single-phase cubic spinel structures with crystallite sizes in a range of ~32 nm to ~36 nm. The lattice constant increases with increasing Mg content. FESEM images show that synthesized The results of IR spectroscopy analysis indicate the formation of functional groups of spinel ferrite in the co-precipitation process. By i ncreasing Mg2 + substitution, room temperature magnetic measurement shows that maximum magnetization and coercivity increase from ~ 57.35 emu / g to ~ 61.49 emu / g and ~ 603.26 Oe ~ 684.11 Oe (1 Oe = 79.5775 A · m -1) respectively. The higher values ​​of magnetization Ms and Mr suggest that the optimum composition is Co_ (0.5) N_ (i0.4) Mg_ (0.1) Fe_2O_4 that can be applied to high-density recording media and microwave devices.