In order to examine bubble evolution and discuss the effects of bubbles effect on the performance of microfluidic fuel cells, two 1.2-mm-depth microfluidic fuel cells employing 0.1-M H2O2 dissolved in 0.1-M NaOH solution and 0.05-M H2SO4 solution as fuel and oxidant, respectively, with transparent lids having width of 1.0mm and 0.5mm, are fabricated in the present study for both cell performance measurement and flow visualization. The results show that the present cells operating at either a higher volumetric flow or a smaller microchannel width yield both better performance and more violent bubble growth. The bubble growth rate, Q g, in a given microfluidic fuel cell is almost the same at different regions of that cell at a given volumetric flow rate, i.e. 10−5 cm3 s−1 and 5×10−5 cm3 s−1, respectively, for cells having widths of 0.5mm and 1.0mm at Q l =0.05mLmin−1, and slightly increases at higher volumetric flow rates. Furthermore, the present study reports approximately constant values of Q g/C d A at various volumetric flow rates, which are 2×10−2 and 5×10−2 cm3 s−1 A−1, respectively, for cells having channel widths of 0.5mm and 1.0mm. In addition, the 0.5-mm-wide cell has higher cell output and performs more tortuous polarization curve.