Decomposition behavior of rice (Oryza sativa) straw, as one of the monocotyledonous angiosperms, was investigated under the two-step semi-flow hot-compressed water at 230°C/10 MPa/15 min (1st stage) and 270 °C/10 MPa/30 min (2nd stage). Prior to the hot-compressed water treatment, cold-water extraction at 20°C/10 MPa/30 min was performed. It was consequently found that some inorganic constituents and free neutral sugars such as xylose, arabinose, glucose and mannose, which would not be chemically bonded with the plant cell wall, were recovered in the cold-water extractives. On the other hand, in the 1st stage, hemicellulose, pectin and para-crystalline cellulose, whose crystalline structure is somewhat disordered, were selectively hydrolyzed, as well as lignin being partially decomposed. In addition, protein was found to be hydrolyzed and formed into various amino acids. Hydrolysis of crystalline cellulose was, however, observed in the 2nd stage. Some additional decomposition of lignin and protein was revealed at this stage as well. In total, 97.9% of oven-dried extractives-free rice straw sample could be solubilized into cold and hot-compressed water. Various products in the water-soluble portion were primarily recovered as saccharides (hydrolyzed products of the polysaccharides), which were partially isomerized and then dehydrated and fragmented. The 2.1% of residue after the treatment was composed mainly of lignin and a trace of silica.