In this study, nickel-zinc ferrite nanoparticles, with the chemical formula of Ni_0.3Zn_0.7-xCu_xFe₂O₄ (where x = 0.1-0.6 by step 0.1), were fabricated by the sol-gel method. The effect of copper substitution on the phase formation and crystal structure of the sample was investigated by X-ray diffraction (XRD), thermo-gravimetry (TG), differential thermal analysis (DTA), Fourier transform infrared spectrometry (FT-IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The XRD result shows that due to the reduction of Zn content,the crystallite size of the sample increased. The results of the vibration sample magnetometer (VSM) exhibit an increase in saturation magnetization value (Ms) for samples with x ≤ 0.3 and a linear decrease for samples with x > 0.3. The variation of saturation magnetization and coercivity of the samples were then studied.