As a work successive to AlCl3 catalytic isomerization of tetrahydrotricyclopentadiene (THTCPD) [Wang, L.; Zhang, X.; Zou, J.-J.; Han, H.; Li, Y; Wang, L. Energy Fuels 2009, 23 (5), 2383−2388], the reaction using chloroaluminate ionic liquid (IL) was investigated. It is found that IL catalysis gives totally different product distribution. The endo-cycloproyl fragments of THTCPD that cannot be isomerized by AlCl3 catalysis are transferred to exo conformation easily. Furthermore, the hydrocarbons are transferred to diamondoids, including methyl-1,2-tetramethyleneadamantane, methyl-diethyl-adamantane, and methyl-diamantane via skeletal rearrangement, with the first diamondoid as the primary and dominant product (selectivity > 80%). The IL shows much higher activity than superacid CF3SO3H, attributed to the synergetic effect of strong acidity and the novel solvent environment. Increasing the temperature, IL dosage, and AlCl3 in IL can promote the rearrangement rate with a slight decrease in the selectivity of methyl-1,2-tetramethyleneadamantane. The presence of trace water in IL forms superacid and induces considerable cracking byproducts, and solvent typically used in rearrangement decreases the reaction rate. The diamondoid-based product shows a low freezing point and high hydrogen content and, thus, is superior as a high-energy-density fuel. This work may open a door for facile synthesis of diamondoid fuel.