In　this　paper,　the　behavior　of　thermal　instability　in　power　MOSFET　is　investigated.　It　is　shown　that　the　device　is　at　risk　of　thermal　instability　when　the　current　has　a　positive　temperature　coefficient.　The　MOSFET　current　has　a　positive　temperature　coefficient　when　gate　voltage　is　less　than　temperature　compensation　point.　The　model　and　experimental　data　show　that　the　sufficient　condition　for　the　thermal　instability　in　the　device　is　S＞1.　The　drain　voltage　has　a　large　effect　on　the　thermal　instability　of　the　device,　and　the　device　with　a　large　transconductance　has　poor　thermal　stability.　Finally,　the　thermal　images　demonstrate　that　even　at　a　steady-state　temperature　field,　there　is　no　uniform　temperature　and　current　on　the　surface　of　the　chip,　and　the　difference　is　greater　in　the　unstable　region.　The　results　confirm　that　the　burnout　of　chip　is　caused　by　local　hot　spot.　©　2019　IEEE.