The values of the magnetostriction constant, λ_s, of Co‐based glass‐coated microwires have been investigated by using the small‐angle magnetization rotation (SAMR) method. Performing the systematic measurements, we were able to choose the appropriate measurement conditions achieving a high‐precision result. From the dependence of the evaluated magnetostriction coefficient on annealing conditions, we evaluated the influence of the internal stresses on the magnetostriction coefficient value and the influence of the heat treatment on the magnetostriction coefficient of nearly zero magnetostrictive Co‐based microwires. We observed changes of the magnetostriction value and sign after annealing. The maximum on the dependence of the magnetostriction coefficient on annealing time is explained considering superposition of the stress relaxation and ordering and beginning of the crystallization process.