PURPOSE: To obtain a treatment agent of oil tank sludge of heavy oil or crude oil for effectively lower the viscosity of said sludge and enabling the easy recovery of a valuable oil component by oil-water separation, by using specific fatty acid diethanolamide.

CONSTITUTION: Fatty acid diethanolamide represented by formula (wherein R is at least one 7W15C alkyl group) is diluted by a straight chain or aromatic hydrocarbon solvent to be dissolved therein and the resulting solution is mixed with oil tank sludge in an amount of 100W20,000ppm as the concn. of fatty acid diethanolamide. After mixing, sludge immediately generates the lowering in viscosity and is easily discharged from a tank by a pump. The discharged treated sludge is allowed to stand to rapidly perform oil-water separation and the aqueous phase is discharge to enable the easy recovery of the oil layer. This treatment agent is especially useful for sludge containing a relatively large amount of moisture and an aqueous sludge component.