The small fuel cell is being researched as an alternative power source to the Li-ion battery in mobile phone. In this paper, a direct hydrogen fuel cell system which powers a mobile phone without a supplementary battery is compactly integrated below 25ml volume at the backside of the phone. The system consists of a small (8ml) metal hydride hydrogen storage tank with 4L hydrogen storage or an energy density of ∼640Wh/L, a thin air-breathing planar polymer electrolyte membrane fuel cell (PEMFC) stack (13.44cm2 ×3mm for a volumetric power density of 335W/L), miniature pressure regulator, and a high efficiency DC–DC voltage converting circuitry. The hydrogen storage tank is packed with the AB5 type metal hydride alloy. The eight-cell air-breathing planar stack (8ml) is very thin (3mm) due to a thin flexible printed circuit board current collectors as well as a unique riveting assembly and is capable of a robust performance of 2.68W (200mW/cm2). A miniature pressure regulator is compact with fluidic and electrical connections within 4ml. A miniature DC–DC voltage converter operates at an overall efficiency of 90%. Consequently, the estimated energy density of a fully integrated fuel cell system is 205Wh/L (70.5Wh/kg).