The form character and oblate shape of cross-section of the yarn in woven fabrics are discussed. Various fabric samples are made by using 106 tex cotton yarn as warp and weft, with changing warp densities and fabric structures as plain、1/2 twill and 1/3 twill respective. The diameter d and horizontal diameter a of the yarn are measured by analyzing the images of the fabric slice. Then the stave extension width coefficient δ of yarn cross-section is calculated. The result shows that there are three different kinds of cross-section shapes of the yarn in fabric as ellipse or similar convex lens, similar to egg shape and similar to shoulder squareness. The more complex of the weave structure, the more kinds of cross-section shape of the yarn has. The yarn cross-section presents different oblate shapes along with the changing densities of interweaving in area A, area B and area C.