Rearrangement of three types of fused polycyclic hydrocarbons with carbon atoms 12–14 was conducted using chloroaluminate ionic liquid (IL) as acid catalyst. The hydrocarbons undergo quick configurational isomerization and then skeletal rearrangement toward alkyl-adamantanes. Although having different molecular structure, these hydrocarbons all lead to adamantanes substituted with 2–3 methyl and/or ethyl groups. The alkyl-adamantanes show high density, low freezing point and viscosity, and are very attractive as high-density fuels. Computation using density functional theory confirms that alkyl-adamantanes are thermodynamically favored and explains the experimental product distribution very well. On this basis, the reaction pathway of each hydrocarbon was illustrated. The reaction conditions including acidity (AlCl3 fraction), temperature, and IL dosage show significant effect on the rearrangement. Moreover, the distribution of alkyl-adamantanes can be tuned by adjusting the reaction conditions, allowing the fuels’ properties to be fine-tuned. This work provides a versatile route to synthesizing alkyl-diamondoid fuels using simple chemical feedstocks in an effective way.