A series of dicarboxylic acid ionic liquids (DAILs) were synthesized from the neutralizing reaction of variable dicarboxylic acids with tetrabutylammonium hydroxide. The ionic liquids (ILs) exhibit good performance in terms of corrosion, viscosity temperature characteristic, and thermal and hydrolysis stabilities. Tribological test results indicated that these DAILs possess superior behavior in reducing both the friction coefficient and wear volume compared with monocarboxylic acid ionic liquids (MAILs), especially when used on steel copper and steel aluminum friction pairs. The enhanced lubricating property is attributed to the two introduced carboxyl groups that enable DAIL molecules to adsorb more efficiently on sliding metal surfaces. Toxicity test data also show that DAIL lubricants possess lower toxicity than MAILs.