Using a simple greener and environmentally free method for the synthesis of cuprous oxide nanoparticles (Cu2O NPs), the effect of Hydroxypropyl-β-cyclodextrin (HP-β-CD) as a capping agent has also been analyzed in detail. In order to produce the NPs, the copper sulfate in aqueous solutions is reduced with L-ascorbic acid and capped with HP-β-CD. Energy-dispersive X-ray spectroscopy (EDX) and X-ray Photoelectron spectroscopy (XPS) are used for the elemental analysis whereas field emission scanning electron microscopy (FE-SEM), and high-resolution tunneling electron microscopy (HR-TEM) are carried out for the analysis of surface morphology of the prepared NPs. In the typically selected area electron diffraction (SAED) pattern along with the lattice fringes, the crystalline structure of NPs could be described as face-centered cubic (FCC) with cu (111), (200), and (220) lattice planes. In addition, HR-TEM images are evident for the Cu2O NPs and revealed that the spherical with an average size of 33.2 nm. In-vitro cytotoxic ability with the NPs has been tested on the MDA-MB-231 (human breast cancer cell) cells and it was revealed to attain nearly 50% toxicity in 2.0 mg/ml (50% cell survival) and other concentrations attained 90-98% survival and a similar pattern is expressed in FACS analysis.