In the current work, drying characteristics of thickened sludge was investigated with respect to drying conditions in oil vacuum evaporation system. The drying characteristics were confirmed by measuring the amount of condensate with 1-min interval. Results showed that the type of mixing oil changed thermal characteristics. The evaporation rate and drying efficiencies of waste cooking oil showed better results rather than other refined oils. From the TG and DTA test results, the ignition temperature varied depending on oil type. The ignition temperature of drying sludge with refined oil was lower than that with waste cooking oil and raw sludge. The moisture content of dried sludge mixed with oil was less than 3%, however, the sludge dried without any oil showed much higher moisture content of 53.7%. This study showed that sludge-derived fuel from the newly developed oil vacuum drying process can be widely used as a renewable energy source.