Cobalt oxide nanoparticles (Co3O4 NPs) were successfully synthesized using ethanolic extract of Trigonella foenumgraceum L. (fenugreek) leaves as a green, potentially low cost, and easily biosynthesized method. The organic bioactive compounds present in fenugreek leaves extract acted as both reducing agents and stabilizing agents for synthesizing metal NPs from cobalt chloride hexahydrate as a precursor. As evidence from UV/Visible spectroscopy, energy dispersive spectroscopy (EDS), and X-ray diffraction analysis (XRD) studies, high alkaline pH was found favorable for the preparation of pure and crystallized single-phase Co3O4 NPs. The interaction of biomolecules from fenugreek leaves extract with Co3O4 NPs was defined by Fourier transform infrared spectroscopy (FTIR) analysis and X-ray photoelectron spectroscopy (XPS). The hydrodynamic size and surface charge of the biosynthesized NPs were measured using light-scattering (DLS) and zeta potential analyses; revealed the formation of negative charged Co3O4 NPs with uniform hydrodynamic size distribution. According to transmission electron microscopy (TEM) analysis, quasi-spherical Co3O4 NPs were synthesized with an average size of 13.2 nm under the modified condition of pH 12 and reaction time of 2 h through inexpensive, environmental friendly benign synthesis process without the use of any additional toxic chemical. (C) 2020 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.