Electrochemical reduction of CO(2)into valuable chemicals is a significant route to utilize CO(2)resources. Among various electroreduction products, oxalic acid (H2C2O4) is an important chemical for pharmaceuticals, rare earth extraction, and metal processing. Here, an aprotic aromatic ester-functionalized ionic liquid (IL), 4-(methoxycarbonyl) phenol tetraethylammonium ([TEA][4-MF-PhO]), was designed as an electrolyte for CO(2)electroreduction into oxalic acid. It exhibited a large oxalic acid partial current density of 9.03 mA cm(-2)with a faradaic efficiency (FE) of 86 % at -2.6 V (vs. Ag/Ag+), and the oxalic acid formation rate was as high as 168.4 mu mol cm(-2) h(-1), which is the highest reported value to date. Moreover, the results of density functional theory calculations demonstrated that CO(2)was efficiently activated to a -COOH intermediate by bis-active sites of the aromatic ester anion via the formation of a [4-MF-PhO-COOH](-)adduct, which finally dimerized into oxalic acid.