There has recently been a wide range of trends in the use of ionic liquids (ILs) as reaction solvent for various polymerization processes. A series of cationic polymerizations of styrene in 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF₆]) had been achieved using a variety of coinitiators combined with a,a-dimethylbenzyl chloride (CumCl) as a initiator at −15 °C. Experiments were carried out to observe the influence of solvent polarity and viscosity on polymerization rate. Compared with those in organic solvents, the cationic polymerization of styrene in [Bmim][PF₆] proceeded in a milder exothermic manner. The end-group structures of the polystyrenes were identified by ¹H NMR spectroscopy and MALDI-TOF-MS. The corresponding elementary reactions and mechanism of styrene cationic polymerization in [Bmim][PF₆] were also discussed.