Giant magneto-impedance (GMI) effect in zero-magnetostrictive Co-based amorphous ribbons samples in their as-quenched and stress-released states as well as with intentionally induced magnetic anisotropy were investigated. Magnetic and impedance properties of the samples exhibiting different anisotropy were compared and the optimum operation conditions for the studied samples from the view-point of their utilization as a sensor element have been determined. A design of a model of magnetic field sensor and characteristics of the constructed prototype are presented.