Alkylnaphthalenes as one of the important lubricating base oils were generally produced by alkylation of naphthalene with olefins, halides, or alcohols. Herein, a totally different alkylating reagent of decalin was studied to react with naphthalene for producing cycloalkylnaphthalene base oils by using the ionic liquid Et3NHCl-AlCl3 as the catalyst. The success for alkylation is attributed to the high reaction activity of the tertiary C–H in decalin under an ionic liquid catalyst. The effect of the two isomers of decalin and reaction conditions on alkylation results was investigated in detail, and it was found that cis-decalin exhibited much higher reactivity than trans-decalin in alkylation. The structure and component of synthetic fluid were confirmed, and their physicochemical properties were tested. Tribological test results demonstrated that cycloalkylnaphthalene exhibits superior friction reduction and antiwear properties compared with commercial alkylated naphthalene base oil AN8.