The hydrothermal processing of fuel oil produced from municipal waste plastics (kerosene fraction: Cl content = 27 ppm, N content = 927 ppm) was investigated under subcritical conditions so as to remove the organic nitrogens contained in the fuel oil. The hydrothermal processing was carried out in a small SUS316 stainless steel continuous packed-bed reactor (packed with Raschig rings) under liquid-phase conditions. Although the denitrogenation took place in pure water, the reaction proceeded much more readily in aqueous solutions of NaOH. That is, the nitrogen content in the product oil decreased to 73 ppm upon processing with 0.05 mol/L NaOH for 4.0 min at 275 degreesC and at the reactant weight feed ratio of unity, whereas it decreased to 16 ppm when 0.10 mol/L NaOH was similarly used at 325 degreesC. The chlorine content in the product oil also decreased to 3 ppm as a result of the processing at 275 degreesC. The rate of hydrothermal denitrogenation could well be described in terms of the homogeneous first-order reaction kinetics with the equilibrium nitrogen content under the hydrothermal processing conditions employed. Upon hydrothermal processing at 275 degreesC, the density, kinematic viscosity, and ignition point of the fuel oil decreased slightly.