Electronics　cooling　has　become　a　key　factor　for　improving　the　performance　of　electronic　devices.　An　effective　thermal　spreader　can　achieve　a　more　uniform　heat　flux　distribution　and　thus　increase　heat　dissipation　in　heat　sinks.　Two-phase　thermosyphon　is　highly　effective　thermal　spreader.　In　order　to　observe　boiling　and　condensation　phenomena,　a　transparent　two-phase　thermosyphon　was　prepared　for　observation　and　study.　The　characteristics　of　phase　change　heat　transfer　were　experimentally　investigated.　The　performance　of　the　two-phase　thermosyphon　with　different　working　fluids　was　measured　for　different　heat　fluxes.　The　experimental　results　show　that　it　has　the　ability　to　level　temperature　and　produces　a　very　uniform　temperature　distribution　in　the　condensation　surface.　The　impairment　of　cooling　condition　on　the　external　side　of　the　condensation　plate　worsens　the　performance　of　the　two-phase　thermosyphon.　Throughout　the　tested　heat　flux　range　in　our　experiment,　the　two-phase　thermosyphon　with　water　as　working　fluid　has　a　better　performance　than　that　with　ethanol　as　working　fluid.　We　also　studied　the　ability　of　the　grooved　evaporation　surface　to　increase　boiling　heat　transfer.　Our　experiments　prove　that　the　two-phase　thermosyphon　with　a　grooved　evaporation　surface　has　a　much　better　performance　due　to　the　increased　heat　transfer　at　the　evaporation　surface.　By　comparing　the　thermal　resistance　of　a　solid　copper　plate　to　that　of　the　two-phase　thermosyphon,　it　is　suggested　that　the　critical　heat　flux　condition　should　be　maintained　if　two-phase　thermosyphon　is　to　be　used　as　efficient　thermal　spreaders　for　electronics　cooling.　(C)　2007　Elsevier　Masson　SAS.　All　rights　reserved.