The synthesis of Fe3O4@ZnS core-shell has been successfully carried out using sonochemical methods. This research was conducted to investigate the crystal structure, functional groups, morphology, and antibacterial activity of Fe3O4@ZnS core-shell by varying the concentration of the ZnS precursor. The XRD characterization results showed that the addition of the ZnS composition resulted in a diffraction peak, representing the inverse cubic spinel (Fe3O4), and cubic sphalerite (ZnS) phases. Based on XRD data analysis, the average particle size of Fe3O4 was 7 - 21 nm, while the ZnS was 3 - 6 nm. The results of FTIR characterization indicated the presence of Fe-O functional groups in the wavenumbers range of 520.24 - 647.36 cm−1 for tetrahedra and 415.83 cm-1 for octahedral, linear to the cubic structure of Fe3O4. In addition, a ZnS functional group appeared at wave number of 959.09 - 1120.25 cm−1 and zinc acetate at wave number of 1548.51 - 1575.79 cm-1. The particle size distribution resulting from SEM characterization for Fe3O4 and Fe3O4@ZnS core-shell with variations of ZnS precursor concentrations of 1x and 2x were 88.36, 105.27, and 156.55 nm, respectively. The antibacterial inhibition of Fe3O4@ZnS core-shell was quite effective in preventing bacterial growth. The diameter of the antibacterial inhibitory capacity of Fe3O4@ZnS core-shell ranged from 3.6 - 7 mm. Thus, Fe3O4@ZnS core-shell is very potential to be applied as an antibacterial agent.