An important point in designing a bolted joint is to know the increment of an axial force produced in a bolt, when a load is applied on an assembly. For this purpose, it is necessary to know the spring constant K_c for a connected part and K_t for the bolt with a nut. In the calculation of K_t, up to now, the deformations of bolt head and nut have not been taken into consideration. A purpose of this paper is to analyze the value of K_t, taking into account these deformation, and to rationalize the design of the bolted joint. Assuming that the bolt head is a finite cylinder and the nut is a finite hollow cylinder, these deformations are analyzed by using the three-dimensional theory of elasticity. Experiments on these deformations are carried out, and the theoretical value is compared with the experimental one. They show a fairly good agreement with each other. Taking into account these deformations, for example, the value of K_t for M24 (clamping length 50 mm) is calculated. It is found that this value is 20% smaller than that of K_t by the conventional method.