Experimental　investigations　of　a　flat　plate　heat　pipes　(FPHP)　are　presented.　The　capillary　structure　is　made　of　intersected　micro-grooves　on　both　evaporation　surface　and　condensation　surface.　The　heat　pipe　manufactured　with　copper　and　filled　with　de-ionized　water,　were　tested　against　heat　fluxes　and　working　fluid　filling　ratios.　The　temperature　fields　and　overall　heat　transfer　resistances　of　the　heat　pipe　under　different　experimental　conditions　are　obtained.　The　measured　temperature　differences　and　the　heat　transfer　resistances　between　the　evaporation　and　the　condensation　surface　of　the　FPHP　are　small,　which　proves　that　the　micro-grooved　structure　greatly　enhances　the　phase　change　heat　transfer.　The　best　performance　of　the　FPHP　is　obtained　at　the　FPHP　working　fluid　filling　ratio　of　87%　and　the　measured　overall　heat　transfer　resistance　is　0.0287　K/W　under　the　optimal　conditions　which　is　even　smaller　than　the　thermal　resistance　of　the　copper　plate　of　the　same　thickness.