Synthesis of exo-tetrahydrodicyclopentadiene (exo-THDCPD) in ionic liquids was studied. Ionic liquids were prepared by aluminum chloride and dialkylimidazolium halide or alkylpyridinium halide and were used in isomerization of endo-tetrahydrodicyclopentadiene (endo-THDCPD) as catalyst and solvent. Effects of reaction conditions such as reaction temperature,reaction time,and ionic liquid amount on the conversion of endo-THDCPD and yield of exo-THDCPD were studied. Results show that the conversion of endo-THDCPD is 100% in different ionic liquids. The yield of exo-THDCPD is 98.3% in 1,3-dialkylimidazolium chloride/aluminum chloride (n(AlCl₃)n( Cl=1.5) at 50 ℃ for 3 h. The yield of exo-THDCPD is 97.8% in 1-butylpyridinium chloride/aluminum chloride (n(AlCl₃)n( Cl=1.8) at 50 ℃ for 3 h. The product can be simply separated and ionic liquids can be reused at least five times without decreasing of the catalytic activity.