The nut deformation, which is the main cause of loosening under repeated axial loads in a bolt-nut unit, is examined analytically and experimentally. A method is proposed to analyze the radial defonnation of the nut, and the deformation of the various nuts is calculated by this method. The validity of the numerical analyses are confirmed by some experiments. Finally, it is shown that the top lace of the nut contracts in radial direction while its bearing surface expands by the force acting on mating thread ridges under tensile load.