Synchrotron X-ray computed tomography has been applied to visualize and quantify the void coalescence leading to ductile fracture in a free-cutting steel. In situ tensile test was carried out and interrupted at different strains in order to observe the sequential damage process. Each void detected by the absorption contrast was sequentially tracked with increasing strain. The results showed that the occurrence frequency of void coalescence was increased exponentially with increasing the strain. Quantitative data obtained by this method was then used to compare and validate the several pre-existing models predicting the void coalescence. Both the Thomason and the Pardoen and Hutchinson models calculated with an average intervoid distance gave a reasonable prediction for large scale coalescence of voids leading to failure.