Single-phase　and　two-phase　flow　distribution　in　plate-fin　heat　exchangers　and　the　influence　of　nonuniform　fluid　flow　distribution　on　the　thermal　performance　of　such　heat　exchangers　were　experimentally　investigated.　The　experimental　results　show　that　flow　maldistribution　can　be　a　serious　problem　in　plate-fin　heat　exchangers　because　of　nonoptimized　header　configurations.　The　uneven　distribution　of　two-phase　flow　in　plate-fin　heat　exchangers　is　more　pronounced　than　that　of　single-phase　flow.　It　is　shown　that　the　uneven　distributions　result　in　a　significant　deterioration　of　the　heat　transfer　performance.　The　relationship　between　the　flow　maldistribution　characteristics　and　the　resulting　loss　in　heat　exchanger　effectiveness　has　been　studied　in　this　work.　Certain　improved　header　configurations　with　perforated　plates　were　proposed　in　order　to　solve　the　maldistribution　problem.　It　was　found　that　the　new　header　configurations　could　effectively　improve　the　thermal　performance　of　plate-fin　heat　exchangers.　By　changing　the　header　configuration,　the　degree　of　flow　and　temperature　nonuniformity　in　the　plate-fin　heat　exchanger　was　reduced　to　16.8%　and　74.8%,　respectively,　under　the　main　test　condition.