The effects of Nb addition on crystallization kinetics and giant magneto impedance (GMI) properties of Fe39Co39Si8B14 amorphous wires prepared by in-water quenching system have been investigated. Thermal behaviors of the wires have been investigated by thermal electrical resistivity measurement and differential scanning calroimetry. The substitution of 4 at% Nb for Fe and Co increases crystallization temperature and merges two crystallization peaks into one peak, leading to a significant increase in thermal stability against crystallization for Fe37Co37Nb4Si8B14 wire. The formation of Fe2Nb phase due to addition of Nb increases the activation energy for crystallization from 425 to 550 kJ/mol. The GMI properties of the alloys are evaluated at driving current amplitude of 10 mA and a frequency of 400 kHz. The alloys show the single peak behavior in the GMI profile. The change in GMI properties increases from 10% at 0 at% Nb to 25% at 4 at% Nb. © 2011 The Minerals, Metals & Materials Society.