Red mud is a by-product of the Bayer process for an extraction of alumina from bauxite ore. The main constituents are Fe203, Al203, TiOe, SiO2 and Na20. Though red mud contains very effective resources as a raw material for the metallurgical industry, it has been concentrated in a large pond and has not been utilized except for limited use as a construction material [1]. We attempted the hydrothermal treatment under acidic and basic solutions for the purpose of recovery of useful resources such as Fe203, TiO2 and A1203. The red mud from the Nippon Keikinzoku Co. Ltd. was used in the present study and it had a very small particle size in the colloidal state. The chemical analyses indicated by weight percentage as follows; Al203: 19.1%, SiO2: 12.8%, Fe203: 40.8%, Na20: 8,4%, CaO: 2.8%, TiO~: 6.4% and the rest was water. The main mineralogical compositions were sodalite, hematite, goethite and anataze. The red mud with the desired solvents was placed into a silver tube (for base) and a fused silica tube (for acid). This tube was placed in a high pressure vessel whose volume was 5.5 cm 3. In the experiment using the fused silica tube, pressure applied to the outside of the silica tube was always higher by about 100kgcm -2 than the internal pressure so as to prevent the fused silica tube from rupturing. The temperature of the vessel was measured by a chromel-alumel thermocouple to the skin of the vessel and held within --!- 4 ° C. The pressure vessel was heated up to 300 to 600 ° C and 700 kg cm -2 for 4 days. When the acid solution whose concentration was 2 to 4 N HC1 and H2SO 4 were used as solvents, the sample, after hydrothermal treatment, separated into two parts. An upper part exhibiting a white colour was composed of anataze crystals and pyrophyllite which is a decomposition product of sodalite under acidic hydrothermal conditions, and the lower part (dark brown) was hematite. The anataze had grown with the habit of a tetragonal prism crystal up to about 40/1m in length as shown in Fig. 1. These results on hydrothermal treatment under acidic solution are summarized in Table. I. On the other hand, Table II shows the results on the hydrothermal treatment under basic conution. All of the sample after hydrothermal treatment had a good sedimentation on account of the grain growth of the very small particles which were in a colloidal state [2]. With increasing temperatures and basic concentrations, the magnetite phase was prominent instead of hematite. All of the