Diethylhexyl adipate (DEHA) has been used for various applications in the chemical industry. In particular, DEHA is used as a plasticizer for polyvinyl chloride and its polymers. In this study, DEHA was successfully synthesized via lipase-catalyzed esterification from adipic acid with ethylhexanol in a solvent-free system. An immobilized lipase from Candida antarctica (Novozym 435) was used as a biocatalyst in this reaction. The optimal reaction conditions for the synthesis of DEHA were temperature of 50 degrees C, an enzyme loading of 5% (based on the total weight of substrate), a vacuum of 6.7 kPa, and a molar ratio 1:2.5 (adipic acid to ethylhexanol). When vacuum was applied, the conversion to DEHA was improved significantly and 100 mol% conversion was obtained within only 3 h under optimal condition.