We investigated the effect of copper substitution on the magnetic and physical properties of Fe3O4 prepared by coprecipitation and flux methods. The coprecipitation compositions were chosen according to the formula (Cux2+Fe1–x2+) Fe23+O4, where $x$ varied between 0 and 1. We found that the flux treatment method is ideal for growing large particles of sub-micrometer size. The size of the final particles obtained was in the range of 200–1500 nm, and the size decreased with increasing copper content. Cubic spinel structured CuxFe3–xO4 particles were obtained by conducting hydrogen gas reduction process at 380 °C–530 °C for 1–2 h after flux treatment. From x-ray diffraction patterns, all particles were determined to be cubic spinel without any sign of the tetragonal structure expected from the Jahn–Teller effect. For these cubic spinel particles, saturation magnetization was controlled at 25–86 Am2/kg, and the value decreased linearly with increasing copper content. The coercive force remained almost constant at 13.7–19.1 kA/m, independent of the copper content.