Spherical and sheet-like Ruthenium oxide (RuO2) nanomaterials were prepared by wet chemical approach. A new kind of sheet-like morphological features in RuO2 (with length and width >1 mu m and 250 nm, respectively) was obtained by using polyethylene glycol (PEG) surfactant. The physical and chemical characterization for the synthesized RuO2 nanomaterials were carried out with the help of field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy. The prepared nanomaterials were assessed for the shape dependent anti-microbial activities against fish pathogenic bacteria such as Vibrio anguillarum, Edwardsiella tarda, Streptococcus iniae and Streptococcus parauberis. Energy dispersive X-ray spectroscopy (EDS mapping) displayed the adsorption of nanomaterials on the bacterial surface. Both types of nanomaterials exhibited higher antibacterial activity against Gram-negative bacteria and in particular, sheet-like RuO2 nanomaterials showed more inhibitive effect than spherical types. (C) 2013 Elsevier B.V. All rights reserved.