Accurate positioning and sizing of defects is a strong requirement for ultrasonic non destructive examination. In complex configurations, this positioning can be greatly improved by using results of simulation. In the case investigated here, the complexity arises from the austenitic cladding of the inspected part: Its surface presents irregularities which may disturb the ultrasonic beam and consequently make difficult the positioning of the defect. In addition, the anisotropy of the austenitic cladding has strong effects on detected echo‐structures. In this communication we present a simulation‐helped method developed in the aim of improving the accuracy of crack tips positioning in French nuclear reactor vessels. We describe this method, based on the CIVA modelling tools, and show results of experimental validation. © 2004 American Institute of Physics