Synergy effects of a basic graphitic-carbon nitride (g-C3N4) supported on a Lewis acidic gamma-Al2O3 (Al@CN) were investigated to remove naphthenic acids (NM) in a highly acidic model crude oil (dodecane used as model component) through a liquid-phase decarboxylation, where a novel bifunctional Al@CN catalyst effectively reduced the acidity of the model crude oil. Among the well-known elimination methods of NM in acidic crude oils, catalytic decarboxylation of NM has been largely attracted due to its simple process scheme by using Lewis acid catalysts. Even though gamma-Al2O3 itself having with mesoporous structures was effective for NM removal, the activity was largely enhanced by an incorporation of the basic g-C3N4 nanoparticles on the Lewis acidic gamma-Al2O3 surfaces at an optimal 20 wt% g-C3N4/Al2O3 through the synergy effects of two active sites. The main roles of the highly dispersed g-C3N4 nanoparticles on the gamma-Al2O3 surfaces were to reduce coke depositions and to adjust the adsorption strength of NAs on the newly formed defected carbon surfaces formed on the Al@CN during a liquid-phase decarboxylation reaction. A proper amount of the thermally stable g-C3N4 species on the acidic gamma-Al2O3 surfaces not only enhanced the catalytic activity by adjusting the acidic and basic sites on the bifunctional Al@CN but also helped in preserving the pristine basic g-C3N4 structures by strongly interacting with the Lewis acidic gamma-Al2O3 surfaces.