In　this　paper,　based　on　the　thermal　conduction　equations　and　the　steady-state　rate　equations,　a　theoretical　and　numerical　analysis　of　thermal　effects　is　investigated　for　a　single　end　pumped　Yb-doped　double-clad　fiber　laser　(YDDC).　The　distributions　of　signal　power　and　temperature　for　different　pump　powers　and　fiber　parameters　are　compared.　According　to　the　results,　the　parameters　of　the　optical-cavity　have　been　optimized　and　an　effective　method　has　been　adopted　to　reduce　the　thermal　effects　in　an　experimental　investigation.　As　a　result,　an　output　power　of　621　W　has　been　obtained　with　a　slope　efficiency　of　78%.　(c)　2007　Elsevier　Ltd.　All　rights　reserved.