||Direct methanol fuel cells (DMFC), using a solid polymer membrane as electrolyte and liquid methanol as fuel, have been projected as promising power sources for portable electronic devices, electric vehicles, and other applications, because this type of fuel cell offers the unique advantages of a lower system volume and weight, a simpler system design, a simpler mode of operation with fast response and better dynamics as well as lower investment and operating costs. The anode flow field is one of the key components of a DMFC, which not only provides channels for liquid methanol fuel to access to the reaction sites, but also allows the removal of gas CO2 generated during the electrochemical reaction of methanol oxidation. Therefore, a better understanding of liquid-gas two-phase flow behavior in the anode flow field is essential for the improvement of cell performance and for the optimal design of an entire DMFC system. This thesis presents a systematic investigation of the characteristics of liquid-gas two-phase flow in the anode flow field of a DMFC. It starts with reporting on an ex-situ study of two-phase flow behavior in an air-water flow system simulating the DMFC anode flow field, in which water was fed into the channel from its entrance, while air was injected uniformly into the channel from a permeable wall. The flow visualization shows that there existed some peculiar flow behavior in such a two-phase flow system, which was never encountered in conventional co-current gas-liquid two-phase flows. For instance, a so-called 'single layer bubbly flow' was found in vertical upward flow, and four transitional flow patterns, namely bubbly-plug flow, bubbly-slug flow, plug-slug flow, and slug-annular flow, were found to exist between the distinct flow patterns. Furthermore, the flow regime maps for various liquid volumetric fluxes have been developed in terms of mass quality versus the volumetric flux of gas phase. An in-situ investigation of two-phase flow characteristics in the anode flow field of an in-house fabricated DMFC was then performed. The innovative design of the experimental setup facilitated a systematic study of various operation parameters, including current density, methanol solution concentration and flow rate, cell operating temperature and cell orientation that affect the CO2 gas bubble behavior, pressure drop, and the cell performance. It has been shown that the two-phase flow patterns and the pressure drop in the anode flow field varied with current density. The experiments have also revealed that the cell performance depended strongly on the cell orientations; the best performance was achieved for the cell to be vertically orientated. More importantly, contrary to conventional conceptions, we find that an external addition of non-reacting gases to the anode flow field of DMFC led to improved cell performance. The theoretical analysis shows that an increase in void fraction of the gas phase in flow channels reduces the cross sectional area of the liquid phase, thereby increasing the liquid velocity. The increased liquid velocity enhances the mass transfer of methanol from the flow channel to the gas diffusion layer and hence, improves cell performance. Following the same idea of accelerating the liquid velocity by reducing channel depth, we have demonstrated that thinning channel from 3.0 to 0.5 mm resulted in an increase in peak power density by 67.5%. Finally, the effects of single serpentine (SSFF) and parallel flow fields (PFF) with different channel sizes on the cell performance and pressure drop were investigated. The experimental results show that the DMFC equipped with the SSFFs yielded better performance than with the PFFs. It has also been found that gas bubbles blocked the flow channels in the PFF at low methanol solution flow rates and high current densities, but this channel-blocking phenomenon was never found in the SSFF under all the test conditions in this work. Studies on the effects of SSFF design parameters revealed that both open ratio and flow channel length had significant influences on the cell performance and pressure drop. The SSFF with either larger or smaller open ratio led to a worse cell performance. However, for the DMFC that operated at low methanol solution flow rates, in the mass transport limitation region, the SSFF with a larger open ratio could enhance the methanol mass transfer, thereby leading to a higher power density. In addition, it was found that for the SSFFs with the same optimized open ratio, a long flow channel was able to give a better cell performance, but caused a larger pressure drop. Keywords: Direct methanol fuel cell; Flow field; Two-phase flow; Flow patterns; Pressure drop; Void fraction; Liquid velocity; Gas injection; Open ration; Flow channel length.