An　experimental　investigation　of　proton　exchange　membrane　fuel　cells　performance　under　different　gravity　conditions　has　been　conducted　in　a　drop　tower.　An　in　situ　visual　observation　of　two-phase　flow　inside　cathode　serpentine　flow　field　of　a　proton　exchange　membrane　fuel　cell　in　normal　gravity　and　microgravity　has　been　performed,　respectively,　while　the　fuel　cell　generated　electric-power.　The　effect　of　gravity　on　mass　transfer　in　proton　exchange　membrane　fuel　cells　was　analyzed　and　discussed.　The　experimental　results　indicate　that　the　liquid　water　accumulate　in　the　bottom　of　the　vertical　flow　channels　in　normal　gravity　condition　and　can　not　discharge　effectively.　The　accumulated　liquid　water　and　reactant　gas　form　the　gas-liquid　two-phase　flow　in　flow　channels.　In　micro　gravity　condition,　however,　the　accumulated　liquid　water　is　pushed　by　gas　and　flows　from　the　bottom　of　the　channels　to　the　outlet.　Removing　accumulated　liquid　water　reduces　the　transport　resistance　of　reactant　gas　(oxygen)　from　channels　to　catalyst　layer,　and　enhances　the　performance　of　proton　exchange　membrane　fuel　cells.