Colloidal silver nanowires become instable and tend to fragment into shortened nanorods and nanoparticles at elevated temperatures. Such morphological variations are associated with the transformation of crystalline structures from the body-centered tetragonal (b.c.t.) lattices into the face-centered cubic (f.c.c.) ones. The crystalline phase transformation has been probed in real time with an in situ technique based on time-resolved high-energy synchrotron X-ray diffraction. Comprehensive analysis of the in situ measurements provides, for the first time, the quantitative understanding of kinetics and thermodynamics involved in the fragmentation of the colloidal silver nanowires.