A novel surface morphology of linear pore pattern on poly(vinylidene fluoride-co-hexafluoro propylene) (PVDF-HFP) flat-sheet porous membrane surfaces prepared by phase inversion technique has been observed. This interesting morphology was repeatedly captured from various casting systems, such as PVDF-HFP/N-methyl pyrrolidone(NMP)/glycol, PVDF-HFP/NMP/diethylene glycol, PVDF-HFP/NMP/glycerol and PVDF-HFP/NMP/glucose, with distilled water as a coagulant. The formation of the linear pore pattern was thought to be associated with the orientation of the macromolecular conglomerations in the casting solution, which occurred during the casting process. Thus, relevant experiments were designed and performed to support the hypothesis. The clearly observed linear pore pattern formed with less than 30 s of exposure time before immersing into water bath became faded in 300 s and completely disappeared in 600 s of exposure time, which was in good accordance with the relaxation degree of orientated macromolecular conglomeration. In contrast, the membrane without experiencing the casting process showed a conventional surface morphology, which also demonstrated the effect of the casting process on the membrane surface morphology. In addition, a clearer image of the linear pore pattern was captured when lower concentration polymer dopes were used. In contrast, this phenomenon was invisible for the membrane formed by a higher concentration dope, which is in agreement with the reduction of orientation degree with increasing polymer concentration. Moreover, the systems containing additives with lower viscosity, such as glycerol, glycol, diethylene glycol and glucose, seem to be easier to form a linear pore pattern than the one containing distilled water, the latter produced a nearly elliptical pore pattern on the membrane surface instead. The investigation on this feature may provide more information for a complete description of the initiation and growth of the porous membrane. (C) 2010 Elsevier B.V. All rights reserved.