The effect of the phosphines for the dimerization of propene using a nickel catalyst system consisting of nickel naphthenate, tertiary phosphine (PCy3, P(i-Pr)3, PEt3, PEt2Ph, and PPh3; Cy = cyclohexyl), triethylaluminum, and 2,4,6- trichlorophenol in the presence of sulfonic acids and/or dialkyl sulfates was investigated. A remarkable increase in both the catalytic activity (turnover number (TON) for the formation of C6 olefins) and the selectivity of dimers (selectivity of dimers based on the propene reacted) was observed, especially upon the addition of sulfonic acids, such as MeSO3H, CF3SO3H, and/or dialkyl sulfates (Me2SO4 or Et2SO4). The reaction products formed by the above catalyst system with PEt3 and PEt2Ph were exclusively C6 olefins (selectivity of dimers: 97 and 93%, respectively). It was also revealed in this catalyst system that the electronic factor of phosphine is important for the catalytic activity, and that the dimer's proportion greatly depends upon the kind of phosphine used.