Phase separation behavior of binary polyolefin blends, ultra-high molecular weight polyethylene (UHMWPE) and liquid paraffin (LP), was investigated through combination of differential scanning calorimetry, rheology and optical microscopy. It was found that the liquid-liquid phase separation (LLPS) could not be differentiated from crystallization in the process of non-isothermal measurements since the LLPS was relatively slow and binodal temperature was quite close to the crystallization temperature. However, by controlling the quench depth rheological dynamic time sweeps and cyclic frequency sweeps showed a clear evidence of liquid-liquid phase separation, which was further justified by optical microscopy. The liquid-liquid phase separation temperature was obtained more accurately by the "inverse quenching" method in rheological time sweeps. Based on the phase diagram, UHMWPE membranes with different porous structures were prepared through thermal induced phase separation with the control of the liquid-liquid phase separation and crystallization. It was found that the LLPS dominated process would produce membranes with the larger porosity and cell size, but relatively smaller tensile strength. (C) 2011 Elsevier B.V. All rights reserved.