The effective magnetic anisotropy of nanocrystalline Fe39.4-xCo40Si9B9Nb2.6Cux alloysHan Xian-Tang1) Wang Zhi1) Ma Xiao-Hua1) Wang Guang-Jian2)The effective magnetic anisotropy 〈K〉 of nanocrystalline Fe39.4-xCo40Si9B9Nb2.6Cux(x=0.5,1,1.5) alloys annealed at different temperatures was investigated based on the law of approach to saturation at room temperature. The experimental results showed that the grain size D of samples with low content of Cu (x=0.5) is larger and a hard magnetic phase precipitated from the residual amorphous matrix at lower annealing temperature which lead to a higher value of 〈K〉. And with increasing content of Cu, which effectively refined the grains and retarded the formation of borides at x≥1,a clear drop tendency of 〈K〉 was observed. The correlation between 〈K〉 and D as well as 〈K〉 and initial permeability μi was also discussed.