In ISO 898-2, the mechanical properties of hexagon nuts are specified so as to prevent the thread stripping fracture mode to utilize 100% loadability of bolts mated, where the proof load of each nut is calculated based on the Alexander theory(1). The validity of the Alexander theory was confirmed to some extent both experimentally and analytically by using FEM(2). However, the significances of three modification factors C1 to C3 in the Alexander equations to modify the effects of nut dilation, bending effects of male and female threads are still not perfectly clear. This study aims to develop a design methodology for hexagon nuts with flange as well as hexagon nuts. Experiments and FE analysis were performed for M 10 style 1 round nuts of same material (equivalent to property class 8) with various geometry including outer diameter (corresponding to width across flats) and the flange diameter. The material property of test nut is determined from the tensile test data with FE analysis by the method shown in Ref (3), and the correctness of theproperty determined is confirmed by the tensile test for the nut with flange (with reduced nut height) and test bolt of property class “14.9” (actual tensile strength Rm=14000 MPa). The results show that the difference of the loadability among these nuts comes from the difference of the shear area caused by the difference of the amount of nut dilation.