Wide-angle X-ray scattering has been used to study the crystalline structure and some structural properties of untreated and thermally treated samples of polycaproamide obtained by hydrolytic and anionic polymerisation of caprolactam. For untreated samples of polycaproamide two crystalline forms exist: a stable α-monoclinic form and an unstable γ ∗-pseudohexagonal modification. There are initial differences in the crystalline parameters of polycaproamide, i.e. content of α- and γ ∗-form, degree of crystallinity and size of lamellae.