As-received CoFeSiB amorphous wires with 128 mu m diameter were electrochemically etched at constant voltage to obtain micro magnetoimpedance (MI) sensor. Wires with different diameters from 13 to 100 mu m were obtained at various constant voltage and pH conditions. The diameters and surface properties of the wires were studied by scanning electron microscopy. It was found that the magnitude of the magnetoimpedance effect was first increased and then decreased with decreasing wire diameter. The thinnest wire obtained at pH 4 and 0.4 V, has a diameter of 13 mu m and this wire shows a 4.95% variation in the magnitude of (Delta Z/Z)(%). The results were discussed On the basis of internal stress distribution and a simple magnetic moment rotational model. (c) 2005 Elsevier B.V. All rights reserved.