In this study, the author has investigated the polymerization of styrene (St), methyl methacrylate, and methyl acrylate initiated with guanidinium-based ionic liquids such as guanidinium tris(pentafluoroethyl)trifluorophosphate ([gua]e3fap), guanidinium trifluoromethanesulfonate, N,N,N ' N '-tetramethyl-N ''-ethylguanidinium tris(pentafluoroethyl)trifluorophosphate, and N,N,N ' N '-tetramethyl-N ''-ethylguanidinium trifluoromethanesulfonate to elucidate the new possibility of ionic liquids as an initiator of radical polymerization. For example, the polymerization of St initiated with [gua]e3fap proceeded at 70 degrees C under air and under argon. It was strongly suggested that the St-polymerization initiated with [gua]e3fap proceeds via a radical pathway, because the inhibitory effects of 2,6-di-tert-butyl-p-cresol and hydroquinone as a radical inhibitor and 1-dodecanethiol as a radical chain transfer agent were observed.