Ultraviolet(UV) irradiated polymerization is a new way to synthesis of the polymeric nanoparticles due to its security, environmental protection, and low investment, as compared to the thermal-initiated polymerization. In this paper, polystyrene-g- poly(N-isopropylacrylamide) (P(St-g-NIPAAM)) nanospheres were prepared by photo-initiated dispersion copolymerization of St and macromonomers, PNIPAAM with a vinyl end group, in a mixture of ethanol and water using 2,2-dimethoxy-2-phenylacetophenone (BDK) as the photo-initiator. The obtained nanospheres were characterized by scanning electron microscopy (SEM) and laser light scattering (LLS). The results showed that the P(St-g-NIPAAM) particles were highly spherical and monodisperse. The size of the nanospheres could be controlled by changing the concentration of the macromonomers, polymerization temperature, and irradiation time and energy.