A dicationic ionic liquid C6(mim)2−HSO4 was synthesized and used as the catalyst for bio-oil upgrading through the esterificartion reaction of organic acids and ethanol at room temperature. The reaction system turned into two layers when the reaction was complete. No coke and deactivation on the catalyst were observed. The yield of upgraded oil was about 49%, and its properties were significantly improved. The higher heating value approached to 24.6 MJ/kg, the pH value increased from 2.9 to 5.1, and the moisture decreased from 29.8 to 8.2 wt %. The chemical composition of the upgraded bio-oil was analyzed with GC-MS. The results showed that organic acids were successfully converted into esters. Thus our data indicated that the dicationic ionic liquid was a promising catalyst for esterification to upgrade bio-oil.