Hollow-fiber membranes were prepared using cellulose acetate butyrate (CAB) as a membrane material via thermally induced phase separation (TIPS) and the effects of preparation conditions (air gap and additives) on membrane characteristics were studied. The presence of the air gap was found to significantly affect the water permeability and microstructure of the hollow-fiber membrane. The hollow-fiber membrane prepared at an air gap of 0 mm has a microporous outer surface, while that prepared at an air gap of 5 mm has a skin layer close to the outer surface. As a result, the membrane prepared at an air gap of 0 mm showed high water permeability and relatively low mechanical strength. The membrane prepared at an air gap of 5 mm exhibited the opposite properties. Of the additives investigated the addition of poly (ethylene glycol) increased the porosity of the membrane surface, while polymeric surfactants (Tetronic (R) 1307 and Pluronic (R) F127) made the membrane surface hydrophilic.