A surfactant is obtained by treating a mixture of 20 to 95 wt. % of a thermal cracking oil fraction obtained by thermally cracking a petroleum heavy residual oil at 400.degree. to 700.degree. C., for example, by delayed coking and 80 to 5 wt. % of other aromatic source, for example, an aromatic fraction having a comparatively high boiling point with an acid catalyst, and sulfonating and neutralizing the product. This surfactant is used as a dispersing agent for a coal-oil mixture (COM).