Radical cation stabilities and their dimerization reactions have been studied through the series of regioregular oligothiophenes (n = 2-6 where n is the number of thiophene rings) with chlorine protected at one alpha-terminal position. The oligomers with two and three rings undergo one oxidation reaction to produce the radical cations, oligomers with four, five and six rings undergo two stepwise oxidations to produce the radical cations and dications, respectively. The radicals coupled to form dimers in following reactions. Based on double-potential-step data we observed that the dimerization rate decreases as n is increased. This result suggests that the stability of the radical increases with increasing size of its oligothiophene chain.