The　temperature　dependence　of　initial　permeability　(mu(i)-T　curve)　for　as-quenched　and　annealed　Si-rich　(Fe0.9Co0.1)(74.5)Nb2Si17.5B5Cu1　alloy　was　investigated.　The　results　showed　that　fewer　Co　doping　into　Fe74.5Cu1Nb2Si17.5B5　can　reduce　the　onset　primary　crystallization　temperature　T-x1,　enhance　the　Curie　temperature　of　amorphous　phase　T-c(am),　and　improve　the　high-temperature　soft　magnetic　properties.　After　annealing　at　480-650　degrees　C,　the　common　feature　of　mu(i)-T　curve　is　that　the　mu(i)　increases　with　T　from　room　temperature　and　reaches　a　peak　value　at　a　certain　temperature,　which　depends　on　the　annealing　temperature　T-a,　then　mu(i)　decreases　with　T.　This　kind　of　mu(i)-T　curves　is　different　from　that　of　the　common　Co-contained　Finemet-type　alloys　ever　reported.　The　optimal　high-temperature　magnetic　softness　was　obtained　by　annealing　at　540　degrees　C,　the　higher　mu(i)　of　6500　at　10　kHz　can　be　kept　up　to　500　degrees　C.　Such　a　magnetic　softness　at　elevated　temperature　is　superior　to　that　of　Fe74.5Nb2Si17.5B5Cu1　alloy　and　other　Co-contained　Finemet-type　nanocrystalline　alloys　with　higher　Co　content.　(c)　2015　AIP　Publishing　LLC.