Cellulose acetate butyrate (CAB) hollow fiber membrane was prepared via both thermally induced phase separation (TIPS) and nonsolvent induced phase separation (NIPS) methods. Atomic force microscope (AFM) and scanning electron microscope (SEM) were used to observe the outer surface morphology of the membranes obtained by different preparation methods (TIPS and NIPS) and various preparation conditions. A tapping mode was operated for studying the polymeric membranes when AFM was applied to image the outer surface of a hollow fiber membrane in the air. Both nodular structure and bumpy structure were observed. AFM images have revealed that both the nodular structure size and the roughness of the membrane outer surface were effected by the air gap distance, coagulation bath temperature and composition. Furthermore, the membrane preparation method (TIPS or NIPS) has profound effect on membrane surface roughness.