Direct formic acid fuel cells (DFAFCs) have received considerable attention because they can generate higher power density compared with other direct liquid fuel cells. However, when the generated CO2 bubbles are retained in the anode porous transport layer (PTL), the performance of DFAFCs deteriorates. In this study, power generation characteristics of DFAFCs using two types of PTL, viz. carbon paper and carbon cloth, were investigated. It was found that the maximum current density is about 60 mA/cm2 higher with the carbon cloth than with the carbon paper. The CO2 bubble distributions in anode PTLs were visualized during the operation using X-ray computed tomography, and the visualized images showed that the PTL structure affected the CO2 bubble distribution, and that the transport paths of formic acid through liquid phase are tortured. Thus, it could be concluded that the transport characteristics of CO2 bubbles affect the power generation characteristics of DFAFCs.