Water　flooding　and　flow　dynamic　characteristics　in　the　flow　field　of　a　transparent　proton　exchange　membrane　fuel　cell　(PEMFC)　consisting　of　an　optical　window　were　studied　experimentally.　in　situ　observations　of　the　liquid　water　build-up　and　two-phase　flow　patterns　inside　the　flow　field　were　made.　The　effects　of　cell　temperature,　cathode　flow　rate　and　operating　time　on　the　flow　patterns　and　cell　performance　were　studied,　respectively.　Experimental　results　indicate　that　the　condensation　of　liquid　water　in　the　flow　field　decreases　with　the　increase　of　cell　temperature.　Increasing　cell　temperature　to　enhance　cell　performance　depends　on　two　aspects:　enhancing　the　electrochemical　kinetics　and　activity　of　the　catalyst　and　reducing　condensation　of　liquid　water　to　improve　mass　transport.　Increasing　oxygen　flow　rate　can　remove　more　liquid　water　out　of　the　cathode　flow　field　and　enhance　cell　performance.　The　distinguished　flow　patterns　in　low-Bo,　low-Su　environment　were　observed.　The　different　flow　patterns　of　slug　flow　with　a　clear　phase　boundary,　core-annular　flow　and　a　transition　flow　pattern　were　found　to　depend　on　the　oxygen　flow　rates　(cathode　Reynolds　number).　Experimental　results　also　show　that　the　water　generation　continues　under　operating　conditions　during　the　operation　time.　The　water　droplets　increased　in　size　with　operation　time,　and　some　water　droplets　were　able　to　grow　to　a　size　of　0.7-0.8mm　in　diameter,　comparable　to　the　cross-sectional　dimension　of　the　flow　channels.　Finally,　the　water　droplets　coalescence　formed　water　columns　in　the　flow　channels.　Mass　transfer　limitation　due　to　liquid　water　column　clogging　leads　to　poor　performance　of　PEMFCs.　(C)　2007　International　Association　for　Hydrogen　Energy.　Published　by　Elsevier　Ltd.　All　rights　reserved.