The dimerization of fatty acid methyl ester was investigated using Brönsted–Lewis acidic ionic liquids (ILs) as catalysts. It is found that IL (3-sulfonic acid)-propyl-triethylammonium chlorozincinate [HO 3 S–(CH 2 ) 3 –NEt 3 ]Cl–ZnCl 2 (x=0.64, x: molar fraction of ZnCl 2 , x=moles of ZnCl 2 /(moles of ZnCl 2 +moles of ammonium salt) was of good catalytic performance. Under the optimum conditions m(biodiesel):m(IL) 15:1, biodiesel 15g, reaction temperature 240°C, and reaction time 5h, the yield of product dimeric acid methyl ester was 96.8%. The reusability of IL was good and after it was used five times, the dimeric acid methyl ester yield was still more than 94%. Otherwise, a synergetic effect of Brönsted and Lewis acid sites enhanced the catalytic performance of the IL.