To provide a polyester modified cross-section fiber for weaving and knitting, suitable for producing woven or knitted fabrics of high class giving a spun silk-like fabric hand having a fluffy feeling and excellent in sewability having the effect of preventing the yarn tightening caused by vertical movement of machine-sewing needles during the high-speed sewing operation.

This modified cross section fiber is a polyester drawn fiber having an elongation stiffness of 800-1,000kg/mm2 and has ridges 2 and grooves 3 alternately and uniformly on the fiber surface in the length. The cross sections of individual ridge 2 and groove 3 are rectangular and almost trapezoidal and they continue in the fiber axis direction and the number and the size of the ridge satisfy the formulas, 15≤N≤35, 0.3≤W≤1 and 1.5≤H≤3W (where N is the number of the ridge, W is width of the ridge in μm and H is height of the ridge in μm).