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Two polypropylene resins with approximate weight-average and z-average molecular weight but differing in low molecular weight tail content were used for the fabrication of microporous membrane based on melt-stretching mechanism. The precursor film with row-nucleated crystalline structure, annealed film and cold-stretched film were prepared and characterized. It was found that the existence of the low molecular weight tail resulted in higher lamellar thickness in precursor film and more perfect grown crystalline part during annealing. During stretching under room temperature, within the plastic region in stress–strain curve, these grown parts were pulled out and converted to bridges connecting separated lamellae. Higher content of connecting bridges existed in the cold-stretching film with low molecular weight tail. In addition, the polypropylene resin with low molecular weight tail showed more pronounced stain-hardening behavior in stress–strain curve, indicating crystallization occurrence of stretched tie chains, resulting in more stable connecting bridges and less deterioration to formed pore structures. Therefore, it can be seen that for polypropylene resin with certain high molecular weight chains, the existence of low molecular weight tail is important to the formation and stabilization of pore structure during the fabrication of microporous membrane by stretching. POLYM. ENG. SCI., 53:2594–2602, 2013. © 2013 Society of Plastics Engineers