Gallium-based liquid metals have increasing applications in a wide variety of emerging areas and they are involved more in frontier studies, the energy industry and additive manufacturing production, and even in daily life. When exposed to open air, large amounts of microorganisms may interact with liquid metals. However, the research of the relationship between pure gallium-based liquid metals and bacterial cells is still limited. In this study, the antibacterial properties of eutectic gallium-indium (EGaIn) alloys were tested against the typical Gram-negative bacteria-Escherichia coli and the Gram-positive bacteria-Staphylococcus aureus and the experimental results displayed that the antibacterial rates reached 100%. We also explored the mechanism of the anti-bacterial properties of EGaIn alloys by measuring the surface composition of the EGaIn film and the concentration of dissolved metal ions. The morphology of the bacterial cells showed that the cell growth and division were influenced by exposure to EGaIn. We also found that the synergistic antibacterial effect came along with the production of reactive oxygen species (ROS). Moreover, the EGaIn film showed enhanced antibacterial activity compared to gallium nitrate at the same initial ion concentration in the solution. This study shows the enormous potential of the anti-bacterial effect of liquid metals.