This study aimed to investigate the structural, magnetic, and antibacterial activities of CrxFe3-xO4 ferrofluids. CrxFe3-xO4 nanoparticles were synthesized from iron sand by a co-precipitation method using Cr3+ ion variants (x content) of 0, 0.05, 0.1, 0.15, and 0.2. The prepared nanoparticles were then characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), and Fourier transform infrared (FTIR). The results of XRD analysis showed that the addition of Cr+ ions caused a shift in the diffraction peak, indicating the successful penetration of Cr+ ions into the Fe3O4 octahedral atomic site. The average particle size of Fe3O4 nanoparticles determined by TEM morphology analysis was 13.7 nm. In addition, the magnetic properties of CrxFe3-xO4 ferrofluids analyzed via VSM showed that the saturation magnetization ranged between 6.95 and 42.71 emu/g, while the magnetic moments were in the range of 7.47 x 10(-19)-19.76 x 10(-19) J/T, which varied with the addition of Cr3+ ions. Furthermore, an analysis of the antibacterial activities of CrxFe3-xO4 ferrofluids revealed that bacterial viability decreased with increase in the number of Cr3+ ions. Thus, the CrxFe3-xO4 ferrofluids prepared from iron sand are potentially excellent antibacterial agents.