The orientation and crystallization during melt stretching were characterized, and their influence on the lamellar morphology and stretched polypropylene pore structure was clarified. During melt stretching, the MDR range from 40-200 could be divided into two regions. In region I, MDR below 120, the crystalline morphology transformed from ellipsoid spherulites to lamellae structure, and the orientation, elastic recovery, and lamellar lateral dimension were enhanced. The porosity of corresponding stretched microporons membrane was increased from 37.8-45.5%. In region II, with the MDR increasing to 200, the orientation and lamellae lateral dimension were increased) but the elastic recovery did not change much. The porosity of corresponding microporous membrane was improved further to 60.3%., The long period, crystalline phase thickness, crystallinity, and lamellae cluster size were kept constant within the whole MDR range, but the orientation was improved from 0.23 +/- 0.02 to 0.41 +/- 0.03. Apparently, the orientation induced the increase of lamellar lateral dimension, and it was the main factor deciding the properties of Stretched rnicroporous Membrane.