A bolted joint is subjected to high stress concentration at the thread root because of its complicated geometry involved. Plastic regions are produced around the bottom of thread root, even under relatively low bolt axial force. It is predicted that the plastic deformation thus produced might affect such important mechanical behaviors of bolted joint as load distributions along threads, the relationships between bolt axial force and angle of nut rotation, etc. In many practical applications, bolts are tightened with considerably high clamping force, especially when tightening critical structures. Thus, it is considered that numerical studies taking the effects of plastic deformation into account are extremely important. However, many previous studies have only dealt with a bolted joint under pure tension. When tightening bolted joints with torque control shear stresses are generated in addition to the tensile stress due to bolt clamping force. In this study, mechanical behaviors of bolted joint during tightening with torque control are analyzed by FEM as an elasto-plastic contact problem, where three-dimensional analysis is conducted employing two dimensional model with each node having three degrees of freedom.