The knowledge of the effects due to residual stresses is of great importance in mechanical engineering industry. The welding processes frequently used in mechanical manufacturing is the principal origin of residual stresses. Many studies were carried out to evaluate residual stresses by using ultrasonic methods. The technique is based on the acoustoelastic effect, which refers to the change in velocity of ultrasonic waves propagating in strained solids. Previous studies do not allow to exactly concluding on the possibility of determining the stress values in the heat-affected zone (HAZ) and the melted zone (MZ). The welding process, causes changes of the microstructure in the HAZ and the MZ, which also involve variations of the propagation velocity. These variations can be more important when compared with those induced by the stresses. The superposition of the both effects involves an over evaluation of the stresses in the two zones. This paper presents advances obtained at CETIM in the field of development of an ultrasonic method for stress measurements. In the paper are discussed the results obtained with ultrasonic on steel welded plate, and a comparison is made with stress measurement obtained by incremental hole drilling method and X Ray diffraction. These results are also validated by thermal relaxation of the plates. The paper discusses also the microstructure influence on ultrasonic measurements of stress. In that case, we work now in collaboration with the Ecole des Mines.