In a process for preparing p-dichlorobenzene by nuclear chlorination of benzene and/or chlorobenzene as the starting material with chlorine molecules, chlorination is carried out using aluminum chloride in an amount of 0.1 - 3 millimols per mol of the starting material and phenothiazines such as 10H-phenothiazine--10-carboxylic acid phenyl ester in an amount of 0.1 -- 0.9 mols per mol of aluminum chloride so as to be a chlorination degree in a range of 1.2 - 2.5, by which p-dichlorobenzene can be obtained in a high para-selectivity and a short reaction time.
