In this study, different nano sizes of lead oxide particles are prepared using high-speed planetary ball milling over a range of grinding times (15, 30, 60 and 120 minutes). The characterization of the prepared nanoparticles is carried out using X-ray diffraction (XRD), transmission electron microscopy (TEM), dynamic light scattering (DLS) and Fourier Transform-Infrared spectroscopy (FTIR). The XRD pattern reveals a phase transition from β-PbO to α-PbO with increasing the milling time. The PbO particles milled for 30 and 60 minutes and termed PbO(A) and PbO(B), respectively, are chosen for this research as they display less agglomeration and achieve nano sizes of 78 and 52 nm, respectively. The following stage utilizes PbO(Bulk), PbO(A) and PbO(B) as fillers and incorporates them independently into PS with a concentration of 10, 15, 25 and 35 wt % using roll mill mixing and compression molding techniques to create a novel polystyrene composite (PS/PbO). PS/PbO composites were then characterized by XRD, which shows a decrease in the phase percentage of PS to α-PbO with increasing PbO content. Extensive research of thermal characteristics employing thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) enables the evaluation of decomposition rates and activation energies. The analysis reveals that the addition of bulk and nano PbO particles shifts Tini and T50% to higher temperatures, and reduces the decomposition rate as well as the activation energy. This means that the thermal stability of PS is increased, especially using a small PbO nano size. This study emphasizes the role of particle size in enhancing the thermal properties of polystyrene, allowing it to withstand elevated temperatures in applications such as coating, insulation and radiation shielding.