Reliability evaluation was made on bolts tightened in the plastic region. As the results of a fatigue test, the fatigue strength of bolts tightened by the plastic region tightening method is improved by 15 to 60%. This effect is attributed to the fact that the deformation of tightened bolts by the external force is restrained by the high initial tightening load in the plastic region tightening method. Reusability of bolts gives no practical problems by avoiding the concentration of the plastic strain. Reliability of the plastic region tightening method in its practical application was confirmed through a durability test of bolts tightened by this method in Komatsu new 11 liter diesel engines.