A Brønsted acidic ionic liquid mim•HCl was synthesized as an intermediate by protonating N-methyl imidazole with Brønsted acid donor hydrochloric acid. And a group of dual acidic ionic liquid 1-hydro-3-methylimidazole chloride chloroironinates (1-x)[mim•HCl]x[FeCl3] (where x is the mole fractions of the Lewis acid) was synthesized from the reaction of mim•HCl and a Lewis acid donor FeCl3. The acidities of the group were characterized and compared to each other by FT-IR in virtue of pyridine and acetonitrile molecular probe, respectively. The dual acidic ionic liquids were then employed to catalyze rosin dimerization. A dimeric rosin acid with a softening point of 102.4 oC was obtained using (1-x)[mim•HCl]x[FeCl3](x=0.64) as catalyst, which was 15 oC higher than that without using catalyst. The catalytic activity became weaker after the fifth use.