A study is made of the magnetic properties of polypropylene fibres (PP) subjected to permanent unidirectional deformation at room and higher temperatures of the medium (120°C). It follows from the results that the values of the magnetic susceptibility, measured perpendicular to the axis of the fibres, change as a function of the relative elongation while they express the changes in orientation of the polymer chains of the fibre in the crystallinity and the deformation of the crystal lattice. The magnetic susceptibility first increases with the relative elongation at room temperature; it then passes through a certain maximum and then suddenly drops and again rises. The rise in susceptibility is explained by the gradual increase in orientation of the macromolecules. The sudden drop in susceptibility is connected with the increase in the crystalline ratio in the fibre and with the formation of inter-molecular bonds. Repeated increase in susceptibility occurs as a result of deformation of the crystal lattice and the symmetry of the electron clouds of bonds is therefore disturbed. In the case of deformation at a higher temperature of the medium (120°C) a repeated increase at greater elongation was not observed since the more favourable energy conditions with such elongation prevents the deformation of the crystal lattice. The conclusions reached from measurements of the magnetic susceptibility agree with the interpretation of the results of measuring the density, optical birefringence and the results of X-ray methods.