Thermal　fatigue　resistance　of　plasma　facing　materials　(PFMs)　is　an　inevitable　concern　for　component　lifetime　and　plasma　operations,　since　the　temperature　fluctuations　will　always　exist　in　future　nuclear　fusion　facilities　and　reactors.　Accordingly,　experiments　were　performed　in　the　electron　beam　facility　to　investigate　the　thermal　fatigue　behavior　under　operational　loading　conditions.　The　tungsten　is　investigated　in　its　stress　relieved　and　fully　recrystallized　state　for　a　better　understanding　of　the　thermal　fatigue　process　when　exposed　to　cyclic　heat　loads.　The　heat　loads　range　from　24　to　48　MW/m(2)　and　the　number　of　cycles　increases　from　100　to　1000　times.　The　results　indicate　that　the　thermal　fatigue　damage　(surface　roughening)　due　to　plastic　deformation　strongly　depends　on　the　loading　conditions　and　the　cycle　index.　As　the　power　density　and　the　number　of　cycles　increase,　the　density　of　the　intragranular　shear　bands　in　each　grain　becomes　higher　and　the　swelling　of　grain　boundaries　becomes　more　pronounced.　The　shear　bands　are　generally　parallel　to　different　directions　for　varying　grains,　showing　strong　grain　orientation　dependence.　Additionally,　extruded　flake　structures　on　shear　bands　were　observed　in　these　damaged　areas.　It　found　that　the　shear　bands　are　generally　parallel　to　the　traces　of　(112)　slip　planes　with　the　surface.　The　results　suggest　that　slip　plastic　deformation　represent　the　predominant　mechanism　for　thermal　fatigue　and　a　set　of　schematic　diagram　is　presented　to　explain　the　formation　of　thermal　fatigue　damage　morphology　(extrusion　and　intrusion　structures).　(C)　2016　Elsevier　Ltd.　All　rights　reserved.