In this paper, we have evaluated the sliding and the loosening behavior of thread joints under transverse loading. Firstly, the critical relative slippage (Scr), less than which the thread joints can keep the fastening, is obtained by the cyclic loading tests. Then, this critical relative slippage is estimated according to the theoretically obtained equation considering the bending deformation of bolt and the geometrical constraint condition. The inclination compliance (kw) of the bolt head used in this equation is evaluated by comparing the experimental result with the corresponding analytical one. In consideration of the nonlinearity of kw with respect to the bolt axial tension, the Scr is well estimated by this equation.