This paper presents a study of the effects of seam parameters on the stiffness of woven fabrics with vertical plain seams. To investigate the stiffness of a sewn fabric, bending length and flexural rigidity must be obtained. In this study, the effects of seam parameters such as sewing thread size, stitch density and seam allowance on bending behaviour were evaluated. Cantilever test was used to determine the relationships between the bending properties and seam parameters of the sewn fabrics, It was found when sewing thread size, stitch density and seam allowance of a vertical plain seam on the cotton woven fabric increased, bending length, flexural rigidity and stiffness of fabric also increased. Therefore there is necessity to optimize the seam parameters such as sewing thread size, stitch density and seam allowance in order to decrease the stiffness of the fabric on the garment. The fabrics with the seams with 3 stitch/cm and 2 mm seam allowance and sewing thread with 140 ticket number had lower flexural rigidity and stiffness.