The giant magneto-impedance (GMI) effect of Co_83.2Fe_5.2Si_8.8B_2.8 ribbons at frequencies of <1 MHz was analyzed. To improve the GMI response, a Joule annealing treatment was conducted with a direct current, and the domain structure of the ribbon surface was investigated via magneto-optical Kerr effect microscopy. The annealed ribbons show larger impedance changes under external magnetic fields, and higher field sensitivity is obtained by certain current annealing treatments. The field sensitivity of 418 and 782%/(kA/m) at 0.2 MHz and 0.8 MHz are achieved after annealing at 0.8 A for 20 min. The annealing treatment under direct electric current induces stress relaxation, and domain rearrangement, and the crystallization process gradually increases with the increasing current density, which gives rise to anisotropic reformation. The release of stresses due to Joule heating below the crystallization temperature causes the homogenous distribution of stress induced by rapid solidification and influences the elastic anisotropy, causing the domain structures to become much more regular. The crystallization, along with the precipitation of hard magnetic phases, increases the crystal anisotropy and induces the intense magnetic coupling action. Consequently, the magnetic domains in the annealed ribbons are rearranged with reformed anisotropy by Joule annealing heat and by the transverse magnetic field induced by the current. The irregular domains, with complex anisotropy in the as-cast ribbons corresponding to the weak GMI response, are transformed into regular and strip-like domains, with transverse easy magnetization after annealing at 0.4 A. After annealing at 0.8 A, the domains are further transformed into fine axial fingerprint-like domains, which are much more sensitive to the change in the axial external magnetic field, allowing for the best GMI response. These results indicate that the Joule annealing treatment is an optional method to optimize the soft magnetic properties and the GMI effect of these Co-rich ribbons at low frequencies.