AbstractA structural‐mechanical study of the orientation process in some flexible‐chain semicrystalline polymers has been conducted. By their deformation properties, such polymers may be conditionally divided into two groups: the ultimate draw ratio is 20‐25 for the first group and 6‐10 for the second one, the transformation of the original structure into a microfibrillar one being completed at a draw ratio (λ) of 8‐10 in the polymers of the first type and at a draw ratio of 2‐4 in those of the second type. The low draw ratios characterizing the moment of a virgin neck propagation in the polymers of the second type are considered to be due to a large number of tie molecules in the original su‐permolecular structures. When microfibrils are formed, these molecules are supposed to be included between fibrils.It is shown that a further increase of λ results from the microfibrillar slip. In the polymers of the first type, the slip is, however, relatively easy and is not accompanied by significant changes in the internal structure of microfibrils, because the interfibrillar tie molecules are almost absent in them. In the polymers of the second type, a significant change in the internal cture of microfibrils, associated with a large number of interfibrillar tie molecules, is observed (obliquity of crystallites and an increase of the long period attributed to an increase in the length of the amorphous intrafi‐brjjlar layer).The microfibrillar slip leads to their more dense packing, which results in a sharp increase in the orientation load required for further drawing. This reduces significantly the lifetime of the specimens in high temperature drawing and leads to their fracture by a thermal fluc