The formation of polyacrylonitrile (PAN) nascent fibers in wet-spinning process was studied by KMnO4 back titration method and scanning electron microscopy (SEM). It has been found that the rate of DMSO diffusion was rapid at the beginning of coagulation then decreased and reached balance at 24 s. With coagulation time increase, the nascent fibers became denser and had fewer inner defects gradually, but transverse stripes appeared on the surface at 24 s, and got deeper. The main defects of nascent fibers included transverse stripes, impurity, fish tail, inner microvoids etc. (c) 2007 Wiley Periodicals, Inc.