The purpose of this report is to discuss the behaviour of bolted joints tightened in plastic region. Although, there have been many reports about plastic region tightening method, their method and discussion were based on the classical criterion derived from the rigid-plastic solution for combined loads of axial tension and torsion. Recently, the finite element method is receiving attention in various fields of engineering because of its diversity and flexibility. In this report, a finite element modeling of bolted joint was performed to confirm the validity of the computer program developed for plastic region tightening analysis. The numerically obtained interaction curves are fairy well agreed with those of experiment. The plastic region is initiated at th root of free thread and it spreads to the inner region of free thread portion of bolt. The elastic zone at the cross section of thread and the partial plastic zone at the thread portion characterize the empirical fact that diameter of circular stress area is the mean of root and pitch diameters.