Poly (vinylidene fluoride) (PVDF) hollow fiber membranes were prepared by adding triethyl phosphate (TEP) to the cooling water bath in a modified thermally induced phase separation process. The effect of TEP content in the cooling bath on the porous structure, crystallinity, thermal and mechanical properties of PVDF hollow fiber membranes was investigated. The melting temperature and crystallinity of the membranes were determined using differential scanning calorimetry. The crystalline and cross-section morphology of the hollow fiber membranes were investigated using wide angle X-ray diffraction and scanning electron microscopy. The resulting membrane exhibited a nearly symmetric structure. The results showed that the TEP content in the cooling bath had a crucial role on the membrane formation, which was also confirmed from the morphology and mechanical properties of the hollow fibers. The porosity, average pore size, crystallinity, Young's modulus, max stress, and elongation at breakage of the hollow fiber membranes can be related to the amount of TEP in the cooling bath. Better pore connectivity was obtained in hollow fiber membranes when the weight ratio of TEP to water was 40:60. (C) 2013 Society of Plastics Engineers