UNLABELLED: The nanoparticles of Cobalt ferrite are synthesized using polyethylene glycol as a solvent by the solvothermal method in a surfactant-free condition. Nanoparticles that were synthesized were determined by using various techniques such as Diffuse Reflection Spectroscopy (DRS), X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), and Energy Dispersive X-ray spectroscopy (EDAX). The Scanning electron microscope confirmed the range of spherical nanoparticles in the size of 20-33 nm.An excellent match was observed between the calculated particles size in the X-ray diffraction and electron microscopes results. Furthermore, their antimicrobial efficacy was determined by MIC, MBC, IC50 and disc diffusion method on Gram-negative ( and ) and Gram-positive () bacteria. The results indicated an acceptable bacteriostatic and bactericidal effects of this nanoparticles. Additionally, it was seen that by the increase in the concentration of nanoparticles, their antimicrobial property would increase. BACKGROUND AND OBJECTIVE: In recent years, antibacterial materials have found a special place to avoid the overuse of antibiotics. In this study, the antibacterial effects of CoFeO nanoparticles on were investigated due to their importance as human pathogens in nosocomial infection. METHODOLOGY: In this study, the antibacterial effects of CoFeO nanoparticles such as MIC, MBC, IC50, and disc diffusion method were examined. FINDINGS: According to the results, CoFeO nanoparticles exhibited potent antibacterial activity against the bacteria that were examined, especially . The MBC (Minimum Bactericidal Concentration) of CoFeO nanoparticle on , , , was between 0.12-0.48 mg/ml and MIC (Minimum Inhibition Concentration) on these bacteria detected between 0.06-0.24 mg/ml. The least IC50 determined for with a concentration of 0.061 mg/ml. and identified as the most resistant and sensitive bacteria in the disc diffusion method, respectively.