Abstract: Abstract In order to describe the yarn’s cross-section geometry of the woven plain fabric, it was necessary to study the flattening of the cross-sections of the yarns along their central lines. Designed 9 kinds of sample of fabrics tightness of 34.72%, 43.4%, 52.08%.Taked pictures by using Keyence digital microscope to samples, did non-destructive measurement and saved date with them. Excel and Origin software were used to input and process date, then drew linear graph of the yarn’s flatness and the measured position to analyze the flattening of yarn’s cross-sections and the impact of the tightness of these samples to the degree of flatness. The conclusions were made that the flattening coefficient of the yarn centerline was strong linear correlation, more near the center of point more great flattening, and not affected by covering factor of the fabric under the condition of low tightness of fabric.

Key words: cross-section geometry of yarn, tightness, plain fabric, flattening factor