We studied the performance and CO2 bubble evolution behavior in an in-house fabricated micro-direct methanol fuel cell (DMFC, 1.0 x 1.0 cm in the active area) with the anode flow field consisting of various-sized microchannels down to 400 mu m. We found that the flow field was blocked periodically by elongated gas slugs due to the increased capillary force in microchannels. This transient capillary blocking caused CO2 bubbles to be evolved in the flow field and to be removed from the cell periodically. We further found that with a reduction in channel size, both gas slugs and the residence time of gas slugs in the flow field became longer. As a result, the effective mass-transfer area of methanol solution on the diffusion layer became smaller, causing the cell performance to decline. At the same fuel feed rate, a smaller flow channel led to a higher mass-transfer coefficient. The competition between the favorable effect of the increased mass-transfer coefficient and the adverse effect of the reduced effective mass-transfer area results in an optimal channel size that gives the best cell performance. (c) 2005 The Electrochemical Society. All rights reserved.