In situ synchrotron X-ray computed tomography has been applied to visualize and quantify the void nucleation, growth and coalescence leading to ductile fracture in a free-cutting steel. Uniaxial tensile test was performed and interrupted at different strain levels in order to understand the sequential damage process. Each void detected by the absorption contrast was sequentially tracked with increasing strain. Quantitative data obtained by this method was then used to compare and validate the several pre-existing models predicting the damage evolution. The gap between the void nucleation predictions and the experimental results was widened with increasing strain at high strain regime, because of the restrictive observation of voids and the void coalescence. The models predicting the void growth were also discussed with reassessing the constant α to take the reduction in equivalent diameter by nucleating voids.