Introduction: Hepatic radioembolization is a minimally invasive procedure involving intrarterial administration of radioembolic microspheres for the treatment of liver tumours. In this study, a biocompatible polystyrene (PS) microspheres formulation containing radioactive Samarium-153 (153Sm) was synthesized and tested. The 153Sm emits both diagnostic gamma energy and therapeutic beta radiation, renders the synthesized microspheres an ideal theranostic radioembolic agent for hepatic radioembolization. Methods: First, the 152Sm2O3 (20 – 50%, w/v) was encapsulated in PS microspheres using solid-in-oil-in-water solvent evaporation method. The 152Sm-labelled PS microspheres were then activated to 153Sm (Eβmax = 807.6 keV, half-life = 46.3 hours) via 152Sm (n,γ) 153Sm reaction in a nuclear reactor with a neutron flux of 2.0 x 1012 n.cm-2.s-1. Physicochemical characterization, gamma spectroscopy and in-vitro radiolabeling studies were carried out to study the properties and stability of the microspheres before and after neutron activation. Results: The 153Sm -labelled PS microspheres achieved a nominal activity of 4.0 GBq.g-1 after 6 hours of neutron activation. Scanning electron microscope (SEM) and particle size analysis show that the microspheres remained spherical with diameters within 15 – 60 μm after neutron activation. No long half-life radioimpurities were found in the samples as revealed by the gamma spectroscopy results. The 153Sm-labelled PS microspheres achieved radiolabeling efficiency of more than 95% in saline and blood plasma over 480 hours. Conclusion: A biocompatible 153Sm-radiolabelled PS microspheres formulation has been successfully developed. The formulation achieved desirable properties for theranostic treatment of liver tumours. The formulation is relatively cheaper, easier to be produced and more readily available.