Blast furnace water-cooled slag (BFWS) has been solidified using a hydrothermal processing method, in which the BFWS could be solidified in an autoclave under saturated steam pressure (1.56 MPa) at 200 degrees C for 12 h by the additions of quartz or coal flyash. The tensile strength development was shown to depend on the formation of tobermorite and the packing state of the formed tobermorite in the solidified bodies. The additions of quartz or fly ash were proved to be favorable not only to the formation of tobermorite but also to the transformation of hibschite, the former improving the strength, and the latter deteriorating the strength. The excessive addition of quartz appeared to cause strength deterioration due to the fact that the residual quartz affected the formation of tobermorite in the solidified bodies.