The hydrothermal method was used to prepare manganese oxide (Mn2O3) nanoparticles. Reduced graphene oxide (rGO) powder was sonicated with Mn2O3 forming a homogeneous Mn2O3-rGO nanocomposite. The prepared nanocomposite was characterized by UV–Vis spectroscopy to unravel origin of absorption photons strongly in the visible region that ensues due to charge transfer, Fourier transform infrared (FTIR) result reveal that the catalytic role of Mn2O3 in graphene reduction in sono-nano composites formation, X-ray diffraction (XRD) result also stand out all of structural parameters enhance slight increase for the position non-ideality (δ dislocation density) of atoms in crystallite size (D) and micro strain (ε).Decreasing the average crystallite size 41.13 nm and 37.2 nm for Mn2O3 and Mn2O3-rGO respectively due to high (600 W) ultra-sonication in presence of rGO-sheets. Field emission scanning electron microscopy (FESEM) show up uniform distribution of Mn2O3 and tightly anchored on the surface of the rGO sheets. These different characterization techniques confirmed the formation of the nanocomposite. Molecular quantum mechanical calculations for interaction investigation mechanism between rGO and Mn2O3, were conducted. Two model molecules were built for Mn2O3 and functionalized rGO, and 8 possible interaction mechanisms between Mn2O3 and functionalized rGO were proposed. Density functional theory (DFT) calculations using B3LYP/LANL2DZ basis set were conducted for the studied model molecules. Electronic properties and reactivity were investigated by calculating molecular electrostatic potential (MESP) mapping for model structures. The most active and plausible structure is the interaction of rGO through the OH group at the terminal with Mn2O3 through the Mn atom. It can be concluded that Mn2O3 activates the rGO surface and increases its detection sensitivity and reactivity. [ABSTRACT FROM AUTHOR]