The hygroscopic property of secondary organic aerosols (SOA) formed by ozonolysis of styrene has been comprehensively studied on the basis of self-assembled hygroscopic-tandem differential mobility analyzer (H-TDMA) and the homemade smog chamber. SOA was produced in the dry chamber (RH<5%) in the presence and absence of ammonium sulfate seed particles, and the diameter-based (70 nm, 85 nm, 100 nm) hygroscopic growth factor (GF) of the SOA as a function of time has been measured by using H-TDMA. It is found that the aerosol water uptake increases with reaction time in the experiment for all the SOA with or without seed particles, which indicates that more highly oxidized polar compounds (more hygroscopic) are partitioned into particles according to the reaction time. The effect of seed particles on the hygroscopicity of the aerosols, although limited by the wrap of organic components, is also investigated. And different classified diameters of SOA exhibit similar hygroscopic growth factors, suggesting that the aerosol composition is independent of particle size.