Abstract: Disclosed is a method for partially hydrogenating a monocyclic aromatic hydrocarbon to produce a cycloolefin, comprising reacting a monocyclic aromatic hydrocarbon with hydrogen in the presence of a particulate hydrogenation catalyst comprised mainly of metallic ruthenium, in a reaction system comprising a continuous aqueous phase having the particulate hydrogenation catalyst suspended therein, an oil phase containing the monocyclic aromatic hydrocarbon, and a gaseous phase comprising hydrogen gas, while applying a shearing force to the reaction system at a specific maximum shear rate. By the method of the present invention, not only can a cycloolefin be produced at high selectivity and in high yield, but also the catalytic activity can be stably maintained at a high level for a prolonged period of time.