Crack propagation within thin glass plates under high shock loading is directly observed using a high speed camera. The fractal dimension of cracks and the power-law exponents of the fragment area distributions are investigated as a function of time. Two models of the fragmentation process are proposed: in one case the cracks are net-like, while in the other the cracks are tree-like, and the relations between fractal dimension and power-law exponent are estimated and compared with the experimental results. It appears that at early stages of the fragmentation process the relation is described by the latter case, while at later stages it approaches that of the former case.