The metal expanded mesh is an attractive alternative to be flow field plate of the direct methanol fuel cell (DMFC) for practical applications. This work investigates the structural effects of the stainless-steel expanded mesh used as a flow field in a passive DMFC. Three expanded meshes with different strand widths are tested at various methanol concentrations. Effects of its assembly diversity, in terms of two different mesh surfaces and orientations of the opening mesh, are also explored at both the anode and cathode. The influential mechanisms in the light of reactants and products management are analyzed by use of a visualization method. The mesh with a smaller strand width yields a better cell performance at a lower methanol concentration, which performs worse at a higher methanol concentration. Compared with the traditional perforated flow fields, the expanded mesh is preferred at lower methanol concentrations. The assembly mode combining BU at the anode and BL at the cathode is recommended. The visualization tests at both sides reveal the positive effects of above optimal configuration on the reactants and products management in a passive DMFC.