Solidification of sediment of river has been carried out using a hydrothermal processing method, in which the sediment was first compacted in a mold at 20 MPa, and then hydrothermally cured in an autoclave under saturated steam pressure at 200 °C for 12 hrs. The riverbed sediment could be solidified hydrothermally with tobermorite formation, and the addition of slaked lime was favorable to tobermorite formation. The strength development of the solidified specimen depended on amount of tobermorite formed, and the more the tobermorite, the higher the strength. At addition of slaked lime 20-40 mass%, the flexural strength was about 23 MPa, which could be used as construction materials. As such, the hydrothermal processing method may have a high potential of utilization of riverbed sediments as construction materials.