Numerous strategies are used for preparing nanoparticles. The green preparation method is environmentally friendly, economical, simple, and faster nanoparticle production. Mn3O4 nanoparticles were prepared by a simple green chemical route using Manganese Dichloride Tetrahydrate (MnCl2.4H2O) as a precursor with Olive leaves extract as a stabilizer agent in the presence of Sodium hydroxide at room temperature. The products (Mn3O4 nanoparticles) were characterized and confirmed using Fourier Transform Infrared (FTIR), powder X‐ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), dynamic light scattering (DLS), and measure surface area. The formation of Mn3O4 nanoparticles was confirmed with an average size of 33.424 nm. The XRD measurements confirmed the presence of a crystalline Mn3O4 phase. Mn (OH)2 and Mn3O4 nanoparticles were investigated by FESEM analysis which gave nanoparticles with scale and cubic shape, respectively. The DLS suggested the particle size distribution appeared in two areas. The surface area of Mn3O4 was 2.0623 cm2g-1. The majority of existing pores were with a diameter of 2–90 nm (mesopores and macropores). Therefore; this green chemical method can be used for the preparation of Mn3O4 nanoparticles in a less time-consuming and in an environmentally friendly way.