Microporous polypropylene hollow fiber membranes were prepared via thermally-induced phase separation process. Several affecting parameters on the membrane structure and performance were investigated based on the competitive mechanism of liquid-liquid and solid-liquid phase separations. Melt index of polypropylene influenced the melt viscosity, processability, and spherulite growth. Residence time within the liquid-liquid phase separation region was controlled by adjusting the air duct temperature, and it affected the liquid droplet growth to result in the changes oi flux and rejection. As the take-up speed at the winder increased, the fiber got more stretched, which increased the pore size and porosity of the membrane. The membrane prepared via thermally-induced phase separation was stretched at room temperature for the enhancement of membrane porosity. When it was stretched before the diluent extraction, it was uniformly stretched due to the diluent presence as a plasticizer. The flux increased while the original rejection was maintained. However, when it was stretched after the diluent extraction and drying, it became so brittle that uniform stretching was not achieved resulting in the membrane damage. In this case, the flux remarkably increased but rejection was too much reduced to lose its function as a membrane.