Co-based rapidly quenched amorphous ribbons are soft ferromagnets with well-adjustable functional properties. Their e®ective magnetic anisotropy and magnetization processes depend on the preparation conditions and geometrical parameters. It was found that giant magnetoimpedance e®ect (MI) for the FeCoCrSiB ribbons without heat treatments after relaxation annealing and after stress annealing at 350°C shows critical dependence on the value of the demagnetizing factor. The range of FeCoCrSiB ribbons lengths with a signi¯cant change in properties is considered in a comparative analysis of the experimental results and model calculations. Quantitative analysis of the contribution of the demagnetizing ¯elds and surface magnetic charges are discussed as the main mechanism for reducing the GMI properties of amorphous ribbons. These data are important for the development of the ribbon-based devices for practical applications of microelectronics. © World Scienti¯c Publishing Company.