Dynamic crack propagation in ceramic composites is analyzed numerically. The simulations concern the effects of microstructural morphologies on fracture. The analysis coniders arbitrary phase distributions in the actual microstructures of alumina/titanium diboride (Al2O3/TiB2) composites. The microstructures analyzed have different phase morphologies and different phase sizes over an order of magnitude in length (from 1–2 to 10–20 μm). A micromechanical model that provides explicit account for arbitrary microstructures and arbitrary fracture patterns is developed and used. The approach uses both a constitutive law for the bulk solid constituents and a constitutive law for fracture surfaces. The model is based on the cohesive surface formulation of Xu and Needleman and represents a phenomenological characterization for atomic forces on potential crack/microcrack surfaces. This framework of analysis does not require the use of any fracture criteria. Instead, fracture evolves as an outcome of bulk material response, interfacial behavior, and applied loading. This approach provides a unified and self-consistent treatment of mixed mode fracture. The evolutions of crack lengths in different phases and along interphase interfaces are calculated to track crack growth. The overall local crack speed, defined as the time rate of change of arc length along zigzagging crack paths, is found to reach the intersonic range, i.e., greater than the shear wave speeds and smaller than the longitudinal wave speeds in the constituent phases. The model also allows the energy release rate to be evaluated easily. For the same amount of crack surfaces generated, the average energy release rates for fracture patterns in four microstructures analyzed differ by up to 25%. The results demonstrate that larger TiB2 reinforcements significantly impede crack propagation and increase the fracture resistance of the composites, as indicated by higher average energy release rate values.