In this article, the relationship between the initial microvoids or cavities variations and lamellar structures evolutions of oriented beta-PP cast films along different tensile directions was mainly investigated via In-situ small-angle X-ray (SAXS) and wide-angle X-ray diffraction (WAXD) measurements. The pore formation mechanism, initial cavities were directly originated from the lamellar separation and its specific positions were at the weak boundary between beta lamellae and adjacent amorphous phase, was proposed. Although the pore formation mechanism of both different stretching directions was consistent, the separation process of beta lamellae was greatly different during stretching. Lamellae on beta-transcrystals (beta-TCs) were preferred to be separated along machine direction, while twisted and inter-crosslinked beta lamellae were more likely to separate along transverse direction. Both stretching directions mainly affected the stage of microvoids growth that could be promoted along transverse direction. Moreover, for the sample of stretching along transverse direction, microporous membranes with higher porosity and more homogeneous micropores distribution could be obtained through subsequent biaxial stretching. Therefore, the suitable stretching methods were optimized for beta-TCs to prepare PP microporous membranes with excellent performance.