This study focused on the effect of spinning conditions on the morphology of PAN fiber in the on-line dry-jet wet spinning. The fiber so produced exhibited a very uniform surface structure compared with wet spun fiber. The coagulant composition (the ratio of dimethyl sulfoxide(DMSO) to water) and coagulation temperature proved to be two cardinal facts for determining the fiber structure. The more circular fibers were obtained at low DMSO contents in the coagulation system and at lower coagulation temperature. However, the take-up speed had little effect on the fiber structure.