A novel concept for diesel fuel processing utilizing H2O2 is suggested to obtain the high-purity H-2 required for air-independent propulsion using polymer electrolyte membrane fuel cells for use in submarines and unmanned underwater vehicles. The core components include 1) a diesel-H2O2 autothermal reforming (ATR) reactor to produce H-2-rich gas, 2) a water-gas shift (WGS) reactor to convert CO to H-2, and 3) a H-2 separation membrane to separate only high-purity H-2. Diesel and H2O2 can easily be pressurized as they are liquids. The application of the H-2 separation membrane without a compressor in the middle of the process is thus advantageous. In this paper, the characteristics of pressurized ATR and WGS reactions are investigated according to the operating conditions. In both reactors, the methanation reaction is enhanced as the pressure increases. Then, permeation experiments with a H-2 separation membrane are performed while varying the temperature, pressure difference, and inlet gas composition. In particular, approximately 90% of the H-2 is recovered when the steam-separated rear gas of the WGS reactor is used in the H-2 separation membrane. Finally, based on the experimental results, design points are suggested for maximizing the efficiency of the diesel-H2O2 fuel processor.