This paper aims to investigate the influence of sewing thread properties, sewing process and repeated washing cycles on the strength reduction of sewing threads and seam strength of sewn samples. For this purpose, twenty-four commercial sewing threads with different types and linear densities were selected for the present study. The samples were sewn with specific sewing parameters. Half of the fabric samples were then subjected to 10 washing cycles. After sample preparation, the change in tensile properties after sewing and washing was calculated and the seam strength of the sewn and washed-sewn samples was measured. It was observed that the breaking strength of each parent thread was significantly higher than its corresponding sewn and washed-sewn thread strength. Considering the strength loss after sewing and after washing, it was found that, among the different types of sewing threads, the samples sewn with core-spun threads showed lower strength loss, while the samples sewn with cotton threads showed the highest loss. Moreover, the values of seam strength after washing are higher for PES-Co core-spun thread and mercerized cotton thread than before washing. However, an opposite trend was observed for the samples sewn with polyester spun and PES-PES core-spun thread. High correlations were observed between the parent thread and the strength of the sewn thread, the strength of the washed-sewn thread and the seam strength before washing.