It is known that the flat fiber of high aspect ratio shows the complexed crimp structure in which the wavy crimp is formed at the edge of the flat fiber as found in sea-weed. The mechanism for the formation of crimp structure in the flat fiber of poly (ethylene terephthalate) was studied in this work. The degree of crimp increased with increasing aspect ratio and spinning speed. The birefringence of this com plexed wavy crimped yarn showed the large distribution along the long axis of the fiber cross section. When the fiber shrank by heat treatment, the structural heterogeneity caused the irregularity of shrinkage which developed the wavy crimp.