Polyacrylonitrile (PAN) precursors were prepared by the wet spinning way. The effects of the coagulation conditions, such as coagulation temperature, coagulation ratio, and coagulation concentration, are discussed in detail. While keeping the coagulation bath concentration constant, as the coagulation bath temperature increased, the cross section deviated less from a circular form, and the as-spun fiber diameter decreased. Measurement to the rate of the boundary movement has been calculated depending on the coagulation rate. While keeping the coagulation bath temperature constant, high coagulation bath concentration can cause more coagulant to diffuse into the solution to the polymer precipitated consequently, which led to a faster coagulation rate. The as-spun fiber from high coagulation concentration was compacted than those from low concentration. The character of the formed structure reflected the system mobility and capability to crystallize. Improvement in fiber density in the as-spun fiber resulted in improvements in the tensile strength of the as-spun fiber. (c) 2007 Wiley Periodicals, Inc.