In order to investigate the influence of anneal treatment on the Co-based melt extraction wires, the soft magnetic properties and giant magnet-impedance （GMI） effect of the annealed Co75Fe40Si8B12Nb1 wire are investigated. Samples are prepared using melt extraction technology in vacuum and annealed at 450℃ without magnetic field or with miagntic field either longitudinal or transverse to the axis of wire of strength from 500 Oe to 4000 Oe，respectively. The magnetic softness and GMI response are measured using HP4192 Impedance Analyzer and Lakeshore7407 VSM. Test results show that the circular anisotropy and GMI response are reduced after longitudinal field anneal treatment. GMI profiles are single-peaked and the maximum impedance ratio ΔZ/Z is 131% and field sensitivity is 7%/Oe. The circular permeability and GMI response are improved after transverse field anneal treatments because of the increased circular anisotropy. At higher frequencies，the GMI profiles change from single-peaked to two-peaked after being annealed in strong transverse field. The circular permeability is increased and GMI effect gets stronger with the increase of current frequency and anneal field. The largest impedance ratio of 190% with a field sensitivity of 26%/Oe is obtained for the Co-based transverse field annealed wires at 10 MHz. The correlations between the circular permeability，the GMI effect and the current frequency are discussed in the light of a skin effect model.