An ionic liquid (IL), 1-ethyl-3-methyl-imidazolium tetrafluoroborate ([Emirn] [BF4]), was investigated for its effectiveness on enhancing oil recovery from hazardous crude oil tank bottom sludge using a solvent. A range of solvent (cyclohexane)/sludge ratio (2-8 mL/g), shaking speed (100-400 rpm), extraction duration (10-120 min), and IL/sludge ratio (0.25-1.0 mL/g) were examined. The addition of IL (i.e., 1 mL/g of IL/sludge ratio) increased total petroleum hydrocarbon (TPH) recovery by 9.4% (from 84.4 +/- 2.4 to 93.8 +/- 2.3% at the solvent/sludge ratio of 8 mL/g). An orthogonal experimental design was subsequently applied to optimize extraction conditions. The use of IL (i.e., 0.1 mL/g of IL/sludge ratio) with the solvent yielded higher TPH recovery (above 95%) at shorter extraction duration (10 min), lower solvent/sludge ratio (4:5 mL/g), and lower energy consumption (100 rpm). The recovered oil had similar calorific value but a higher F3 fraction compared with crude oil. The results suggested that the IL-enhanced solvent extraction with lower solvent consumption is an effective approach for oily sludge treatment.