The iron oxide (IO) nanoparticles have many potential applications in biology due  its magnetic properties. In this study, we analyzed antimicrobial activity of two important (IO) nanoparticles (g-Fe₂O₃, Fe₃O₄). These (IO) nanoparticles were characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM) and the starch were used as dispersing agent in order to reinforce the colloidal stability of IO nanoparticles. The DLS results showed the average size of IO nanoparticles were an about 30-40 nm and TEM results demonstrated that IO nanoparticles were well dispersed with starch. Further, Staphylococcus aureus (PTCC 1431), Escherichia coli (PTCC 1395), Pseudomonas aeruginosa (PTCC 1599) and Candida albicans (PTCC 5027) were grown in the presence of five different IO nanoparticles concentrations with and without starch. 3-[4,5-dimethylthiazol- 2-yl]-2,5-diphenyltetrazolium bromide (MTT) reduction  and agar well diffusion assays were performed and the results provide evidence that g-Fe₂O₃ and g-Fe₂O₃/starch nanoparticles did not inhibit  microbial growth even at the highest concentration (50 mg/mL). But Fe₃O₄ nanoparticles inhibited microorganisms growth, specially at the highest concentration (50 mg/mL) and at the present of starch. As a result, starch as dispersing agent can increase the antimicrobial effects of Fe₃O₄ nanoparticles.