An　effective　thermal　spreader　can　achieve　more　uniform　heat　flux　distribution　and　thus　enhance　heat　dissipation　of　heat　sinks.　Vapor　chamber　is　one　of　highly　effective　thermal　spreaders.　In　this　paper,　a　novel　grooved　vapor　chamber　was　designed.　The　grooved　structure　of　the　vapor　chamber　can　improve　its　axial　and　radial　heat　transfer　and　also　can　form　the　capillary　loop　between　condensation　and　evaporation　surfaces.　A　two　dimensional　heat　and　mass　transfer　model　for　the　grooved　vapor　chamber　is　developed.　The　numerical　simulation　results　show　the　thickness　distribution　of　liquid　film　in　the　grooves　is　not　uniform.　The　temperature　and　velocity　field　in　vapor　chamber　are　obtained.　The　thickness　of　the　liquid　film　in　groove　is　mainly　influenced　by　pressure　of　vapor　and　liquid　beside　liquid-vapor　interface.　The　thin　liquid　film　in　heat　source　region　can　enhance　the　performance　of　vapor　chamber,　but　if　the　starting　point　of　liquid　film　is　backward　beyond　the　heat　source　region,　the　vapor　chamber　will　dry　out　easily.　The　optimal　filling　ratio　should　maintain　steady　thin　liquid　film　in　heat　source　region　of　vapor　chamber.　The　vapor　condenses　on　whole　condensation　surface,　so　the　condensation　surface　achieves　great　uniform　temperature　distribution.　By　comparing　the　experimental　results　with　numerical　simulation　results,　the　reliability　of　the　numerical　model　can　be　verified.　©　2008　by　ASME.