Abstract: Nonlinear ultrasonic was used to inspect different plastic damage specimens of welded joint of steam turbine steel 30Cr2Ni4 MoV. Results show that nonlinear parameter increases clearly with the plastic strain or plastic damage of specimens and the variation of nonlinear parameter of weld center is the maximum, whichis compared with the ones of the heat-affected zone and base metal under the same plastic strain condition.Moreover, it is observed that the dislocation density increases with the plastic strain in the weld center in TEM photographs, which is the primary cause to induce the increase of nonlinear parameter with the growing plastic strain.