BACKGROUND AND AIM: The emergence of resistance against antimicrobial agents has led to the development of more efficient agents and new techniques for treatment of various microbial infections. The aim of the present study is to determine the antibacterial and antifungal activity of bare and chitosan coated FeO nanoparticles (NPs) against five organisms, Escherichia coli (E. coli), Bacillus subtilis (B. subtilis), Candida albicans (C. albicans), Aspergillus niger (A. niger) and Fusarium solani (F. solani). METHODS: FeO NPs were synthesised by coprecipitation and surface coating was done by chitosan polymer to avoid agglomeration. The antimicrobial property of NPs was tested by agar well diffusion and analysed by measuring the diameter of the inhibition zone. RESULTS: Average particle size of FeO and chitosan coated FeO NPs was 10.4 ± 4.9 and 11.4 ± 5.2 nm, respectively. Mean diameter of inhibition zone of synthesised chitosan coated FeO NPs was in the range 14.5 to 18.5 mm. The effect of chitosan coated Iron oxide nanoparticles was F. solani/A. niger < C. albicans < E. coli/B. subtilis (p < 0.001). CONCLUSIONS: Chitosan coated FeO NPs are effective antimicrobial agents and so may be developed as a microbial resistant coating for biomedical devices.