Abstract: Disclosed is a cleaning composition useful for dislodging fouling from process lines of oil refining or petrochemical plants and a cleaning method using the composition. The cleaning composition comprises 0.01 to 1 wt % of a C8 aromatic compound, 75 to 85 wt % of a C9 aromatic compound and 14 to 24 wt % of a C10 aromatic compound. When applied to the process lines, the composition is mixed at an amount of 2 to 20 vol % with Light cycle oil or light gas oil. Chemical cleaning with the composition effectively removes the fouling formed within the process lines and heat exchangers of oil refining or petrochemical plants, and recovers the processing capacity of heat exchangers to the start of run level, bringing about a significant economic profit.