A silicon-based micro direct methanol fuel cell (μDMFC) for portable applications has been fabricated and its electrochemical characterization carried out. A membrane-electrode assembly (MEA) was specially fabricated to mitigate methanol crossover. The cell with the active area of 1.625 cm 2 demonstrated a maximum power density of 50 mW/cm2 at 60°C. Since silicon wafer is too fragile to compress for sealing, and a thicker layer of gold has to be coated on the silicon wafer to reduce contact resistance, further development of micro DMFCs for high power application was carried out using stainless steel plate as bipolar plate in which flow channels were fabricated by photochemical etching technology. The maximum power density of the micro DMFC reaches 62.5 mW/cm2 at 40 °C and 100 mW/cm 2 at 60°C with atmospheric pressure. An 8-cell air-breathing DMFC stack has been developed. Mass transport phenomena such as water transport, and oxygen transport were investigated. By using a water management technique, cathode flooding was avoided in our air-breathing DMFC stack. Furthermore, it was found that oxygen transport in the air-breathing cathode is still very efficient. The DMFC stack produced a maximum output power of 1.33 W at 2.21 V at room temperature, corresponding to a power density of 33.3 mW/cm2. A passive DMFC using pure methanol was demonstrated with steady-state output power of 20 - 25 mW/cm2 over more than 10 hours without heat management.