Thermal　management　for　package　assembly,　especially　for　high　power　packages,　is　very　important　to　the　reliability　and　quality　of　electrical　products.　In　this　paper,　a　finite　element　approach　is　proposed　to　investigate　the　thermal　performance　of　T0252-5L(B)　package　attached　on　printed　circuit　board(PCB)　with　different　layers　and　sizes,　which　is　further　related　to　reliability　issue　of　this　type　of　IC　package.　This　single-factor　method　shows　change　trend　of　thermal　resistance　following　materials　and　dimensions　of　package.　Furthermore,　design　of　experiment　(DOE)　approach　is　used　to　find　the　main　factors　and　optimal　combination　of　these　control　factors　for　minimizing　of　the　thermal　resistance　for　packages.　The　simulation　results　show　that　thermal　conductivity　of　adhesive,　die　to　die　pad　ratio,　thermal　conductivity　of　EMC　and　thermal　test　PCB　board　have　significant　impact　on　thermal　resistance　of　TO252.　In　addition,　interactions　of　those　factors　are　also　prominent,　and　the　minimum　thermal　resistance　of　T0252　is　30.46K1W　after　optimization.