Tensile tests on a ultra high strength steel under in situ X ray tomography made it possible to observe and to quantify 3D damage of this material. The analysis of the data gives access to the main features of cavities (density and size) as well as local strains and triaxiality within the whole tensile specimen. Based on these results, a new damage model has been put forward that is noticeable by integrating cavity nucleation into the classical Rice and Tracey approach. The predictions of the new model fit significantly better with the experimental results.