Abstract: This paper reports on the dispersion photopolymerization of polystyrene microspheres by low-power ultraviolet light in vacuum with styrene as initial monomer, 2,2'-Azobisisobutyronitrile (AIBN) as photo-initiator, methanol as solvent, and polyvinylpyrrolidone(PVP) as dispersant. The microspheres were obtained at room temperature by the irradiation of 8 W 365nm ultraviolet light. Instead of N2 protection, the vacuum was first introduced to help the polymerization and it had been proved that only the photopolymerization could be used in vacuum. When UV light irradiated the reaction system, the C-N bond of AIBN could be broken for the absorption of photons and free radicals would form, and then these radicals would induce the polymerization of styrene. The experiments showed that quantity of initiators, quantity of dispersants, quantity of monomers, power of light and reaction time could regularly affect the size and size distributions of the finally obtained microspheres. Tuning these factors properly, for example, keeping the volume ratio between solvent and monomer as 3:1, the weight ratio between initiator and dispersant as 3:1 , after irradiated 24 hrs with 0.71 mW/cm2 UV light, monodispersed microspheres with certain size could be easily prepared. This method is low-cost, simple, safe and universal for the preparation of microspheres. It could be also used to fabricate other polymer microspheres, such as PMMA, PMAA, P(MMA-co-MAA) copolymer microspheres and even the functional organic-inorganic composite microspheres.