Two different sizes of PbO nanoparticles, notably PbO(A) and PbO(B) with sizes of 78 nm and 54 nm, respectively are produced using a high-speed planetary ball milling machine under specific operating parameters. Following, the novel nanocomposite PS/PbO is synthesized using compression molding by embedding 10, 15, 25, and 35 wt% of PbO(Bulk), PbO(A) and PbO(B) into PS separately. The composite is further characterized by Fourier transform infrared spectrophotometer (FTIR), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS) and laser-induced breakdown spectroscopy (LIBS). FTIR results confirm the presence of PbO, and indicate a physical adsorption of the nanoparticle onto the PS matrix surface. SEM, EDS and LIBS analyses reveal a more efficient diffusion of PbO in the PS matrix with the decrease of the nanoparticle size. On the other hand, Tensile and Vickers microhardness tests are performed to investigate the composite's mechanical properties. The stiffness is, indeed, enhanced with increasing weight fraction, as well as with decreasing particle size of PbO. Whereas, the strength of the composite is optimized with 15wt% of PbO. Microhardness test reveals an ISE behavior of the composite, and an increase in H v values with PbO loads up to 15 wt%. Accordingly, by adjusting the filler particle size and concentration, the mechanical properties of the composite are evidently enhanced, increasing their use in a variety of applications such as coating, insulation and radiation shield.