The invention concerns a treatment method which consists in coating at least one surface of a brittle thin metal strip (1) with a coating film (3, 3') comprising at least a polymer material, to obtain on the strip an adhering coat with a thickness ranging between 1 and 100 .mu.m, modifying the working and breaking properties of the brittle thin strip (1); then carrying out a step wherein the brittle thin metal strip (1) is subjected to stresses, such as a cutting operation, on the brittle thin metal strip (1) coated with the film coating (3, 3'). The method is particularly useful for producing magnetic parts from strips or ribbons with nonocrystalline structure.