Although the microporous membrane prepared based on the melt stretching mechanism has been commercialized for more than 20 years, the formation process from the initial lamellae structure to final fiber connecting bridges and pores is still unclear. In this work, to clarify the transformation mechanism, in situ SAXS and WAXS were carried out during hot stretching at 130 degrees C to 100%. The scattering patterns from the annealed film, cold stretched film to 20% (stretched at room temperature), and heating to 130 degrees C were also collected. The preparation technology was similar to that during the commercial fabrication. It was found that during cold stretching to 20% many long and narrow crazes are formed between separated lamellae clusters, and a part of destroyed crystals appeared. After heating to 130 degrees C, oriented structure and needlelike voids appeared, which was related to the shrinkage of oriented amorphous chains along the transverse direction, due to the tension stress effect. Also some oriented crystal structure was formed. During hot stretching to 20%, the lamellae which are close to the craze wall are rotated as the fibril crystal as the axle and the connecting bridges were formed among the separated lamellae cluster. Further stretching to 100%, these connecting bridges transformed to fiber bridges, contributed by strain-induced crystallization. During the whole hot stretching, the amorphous chains oriented along the machine direction and also crystallized into fiber bridges. This is the first time to clearly describe the lamellae to fiber bridges transformation during the preparation of microporous membrane.