This is the first report to model the behavior of phosphorus in sewage sludge under a hydrothermal condition for the ranges of temperature 300-600 degrees C and reaction time 5-30 s using a continuous reactor. The pressure was fixed at 25 MPa. H-2, CO2, and CH4 were components of the product gas. At the short reaction time of less than 10 s, organic phosphorus (OP) was almost completely converted to inorganic phosphorus (IP) under supercritical conditions. Then, a part of IP precipitated in the reactor. Kinetics parameters for the reaction from OP to IP were determined by assuming the first-order reaction. The precipitation rate in proportion to oversaturation fitted the experimental data well.