In this article, the effect of jet stretch ratio on the extrudate die-swell effect of polyacrylonitrile spinning solution and the structure and properties of as-spun fibers was systematically analyzed by means of X-ray diffraction (XRD), electron microprobe analysis, and the measurement of die-swell ratio, boiling-water shrinkage, porosity, mechanical properties analysis, etc. It revealed the formation mechanism of the die-swell effect and spin orientation and its influences on the structure and properties of as-spun fibers. It showed that with the increase of the jet stretch ratio the die-swell ratio became smaller, both the degree of spin orientation and the crystallinity increased, the microstructure of as-spun fibers became compact and homogeneous, and the cross section tended to be circular. As a result, the breaking tenacity of as-spun fibers and resultant precursors all increased. (c) 2006 Wiley Periodicals, Inc.