In order to investigate fragmentation mechanism in thermally tempered glass, crack propagation and crack divergence were observed with high-speed photography using Cranz-Schardin type camera. Tempered glass of 3.5 mm and 10 mm thick and zone-tempered glass of 5 mm were used as specimens. In addition to the conventionally known bifurcation as crack divergence, branching type was also seen. Differences in propagation energy were seen between bifurcation and branching after at once divergence by Caustics method. Bifurcation and branching mechanism were explained using the concept of stress σCR which was introduced for explanation crack connection phenomenon seen in zone-tempered glass. The new crack generated by two cracks colliding also existed, and its collision angle of the two cracks had a major influence on its formation by stress σCR. Stress σCR was not always explained by stress intense factor K1 because it has characteristics of time dependance and not necessarily plastic deformation.
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