A preventive underwater survey conducted in 2008 in the Ljubljanica river near Sinja Gorica (between Vrhnika and Ljubljana, Slovenia), revealed the remains of a sunken vessel. Investigations showed that it belonged to a flat-bottomed barge, at least 16 m long, with nearly vertical side planks connected by iron clamps. We present here the results of wood identification, dendrochronological dating and investigations of wood properties and preservation carried out on timbers from a portion of the vessel. The bottom and side planks of the barge were made of beech wood (Fagus sylvatica). The wood could be dendrochronologically dated using local reference chronologies with a post quem end date of AD 3. The high maximum water content of the wood (690 %), erosion of wood cell walls and degradation of the main cell-wall constituents observed under the microscope using various techniques, indicate poor preservation of the barge wood. Use of beech wood, which has good mechanical properties but very low resistance against decay organisms, and unique coupling with iron clamps indicate that the barge was solidly built. It was probably used for transporting heavy cargo on the Ljubljanica river between the Roman settlements of Nauportus and Emona during the first decades AD. The barge is one of the oldest known Roman ships dated by means of dendrochronology. Its conservation is extremely endangered due to the poor wood preservation. Recommendations for future interventions are proposed.