A series of substituted styrenes were polymerized by controlled/"living" radical polymerization using atom transfer radical polymerization (ATRP), in order to correlate monomer structures with polymerization rate. The effect of substituents is discussed with regard to the Hammett equation. The results show that most of the monomers can polymerize in a controlled way by ATRP; i.e., the apparent polymerization rate is first order with respect to monomer concentration, and molecular weight increase!; linearly with monomer conversion. The molecular weights obtained fit the theoretical values and polydispersities are relatively low (MwMn < 1.5). Monomers with electron-withdrawing (EW) substituents result in better polymerization control and polymerize faster than those with electron-donating (ED) substituents. The apparent polymerization rate constants follow the Hammett equation with rho = 1.5. Further study indicates that the difference of polymerization rates for different monomers can be attributed to both propagation constant, k(p)(.), and the equilibrium constant, K-eq, for atom transfer. Monomers with EW substituents have larger k(p)(.) and K-eq values than those with ED substituents; therefore, EW substituents increase the monomer reactivity and decrease the stability of dormant species, while ED substituents have the opposite effects.