A process for producing a high trans low vinyl conjugated diene (co)polymer, comprising polymerizing at least one conjugated diene alone or in combination with at least one alkenyl compound using a catalyst composition comprising (a) a barium compound, (b) an organoaluminum compound, (c) an organolithium compound and (d) a hydroxyl group-containing organic compound. This process enables the easy production of a high trans low vinyl conjugated diene (co)polymer having a high trans 1,4 bond content, a low vinyl bond content and a high molecular weight, at a high polymerization activity.