The extremely soft magnetic behaviors in the nanocrystalline $Fe_81B_11Nb_7Cu_1$ alloy annealed at 450 $circ C$ and 550 $circ C$ for 1 hour in a vacuum were investigated by means of the magnetoimpedance (MI) effect and the incremental permeability. Because the MI effect can be obtained only in ultra-soft magnetic materials, the improvement of magnetic softness by proper thermal treatment was carefully monitored by the MI effect for all annealed samples. The changes of the incremental permeability as a function of an external field were also measured to verify the magnetic softness along with the MI measurement.