The isomerization of endo-tetrahydrodicyclopentadiene (endo-THDCPD) to exo-THDCPD (Jet Propellant-10) and adamantane was investigated using ionic liquid (IL) as acid catalyst to explore an alternative greener than widely applied AlCl3-based operation. ILs composed of various 1-alkyl-3-methylimidazolium chlorides ([RMIM]Cl) and metal chlorides were checked, and [BMIM]Cl/AlCl3 was the best. The effects of acidity and dosage of IL, reaction temperature and time were studied for endo- to exo-isomerization of THDCPD. The reaction occurs quickly under mild conditions with both conversion and selectivity beyond 98%. Purifying the reactant can suppress deactivation of IL; and the used IL can be recycled four times by heating under vacuum after each run. By adjusting the reaction to severe conditions, i.e. higher IL dosage, higher temperature and longer time, exo-THDCPD is further isomerized to adamantane. But both the conversion and selectivity are much lower than those of endo- to exo- isomerization. The highest yield of adamantane is 50.9%, which can be further improved by some additives like 1-bromoadamantane. Compared with other methods, IL catalysis has many advantages from energy and environmental perspectives.