3,7-Diethyl-10-phenylphenothiazine (DEPPT), a phenothiazine derivative whose 3,7- and 10-positions are blocked, was synthesized. Potentiostatic electrolysis of DEPPT in acetonitrile (ACN) in the presence of 0.1M of LiClO4 at 0.7 V (vs. Ag/Ag/Cl) yielded the stable cation radical of DEPPT (DEPPT(+.)) which was characterized by cyclic voltammetry, UV-visible spectroscopy, and ESR spectrometry. Stable cation radicals of 10-phenylpheno-thiazine and 3,7-diethyl-10-methylphenothiazine were also prepared. The cationic polymerization of n-butyl vinyl ether was initiated by these cation radicals, including DEPPT(.+). The electron transfer mechanism for the initiation step, which we proposed previously, was supported by the fact that DEPPT(.+) was capable of initiating the polymerization; dimerization of DEPPT(.+) by releasing protons is precluded because 3,7- and 10-positions are all blocked.