Measurement of the magnetostriction constant of amorphous wire by conventional techniques is very difficult because of its small diameter. However, accurate determination of the magnetostriction constant is important in the study of amorphous wires. Here the saturation magnetostriction constant (λs) for a low-magnetostriction amorphous wire of nominal composition (Fe6.3Co92.7Nb1) 77.5Si7.5B15 has been determined by means of the small-angle magnetization-rotation method. λs has been evaluated to be 2.1×10-7 for its as-received state. The dependence of thermal treatment is also reported.