In this paper by proposing a novel mechanism, we reveal one of the most controversial issues in the hollow fiber fabrication process regarding the instability leading to the deformed cross-section of fibers fabricated through non-solvent induced phase separation. We have analyzed possible instability mechanisms based on our experimental observations and then postulated that the principal instability occurs in the external coagulation bath where the pressure induced in the nascent fiber outer layer as a result of diffusion/convection, precipitation, densification and shrinkage buckles the rigid precipitated polymer shell in the dope and bore fluid interface. In addition, the effect of some spinning conditions such as air-gap distance, bore fluid composition, take-up speed, external coagulant and dope concentration on the final shape of the fiber cross-section have been investigated. The proposed mechanism is in good qualitative agreement with all our observations. (c) 2007 Elsevier B.V. All rights reserved.