The properties of 100% PES core-spun threads were studied before and after the sewing process at different stitching speeds, by determination of the threads tensile force during the sewing process and by changes of the mechanical and thermo-mechanical properties after the sewing process. This analytical method is especially advantageous when monitoring contemporary thermal and mechanical effects, as is the case when loading the sewing thread. Loadings during the sewing process cause structural changes in the thread-twisted fibres. This is confirmed by changes in the thermo-mechanical properties after the sewing process. Simultaneously, changes in the threads mechanical properties after sewing were also observed.