The　reference　stress　method　(RSM)　is　a　classical　method　to　estimate　C*-integral　of　creep　crack.　An　extended　reference　stress　method　(ERSM)　is　given　for　the　central　cracked　plate　(CCP)　under　biaxial　loading　in　this　paper.　The　applicability　and　verification　for　the　proposed　ERSM　is　given.　The　study　finds　that　the　solutions　with　the　proposed　ERSM　agree　better　than　those　of　RSM　under　biaxial　loading　condition.　A　theoretical　form　to　predict　the　relationship　of　C　*-integral　between　biaxial　loading　and　uniaxial　loading　is　discussed.　Relation　between　C*-integral　and　creep　time　under　biaxial　loading　is　validated　and　discussed.