Abstract: The mode I interlaminar fracture toughness of composite laminates with epoxy interleaf was studied in the double cantilever beam (DCB) tests. Brittle unstable and stable delaminations were observed in the specimens with and without interleaf respectively. For the unstable crack growth, based on dynamic fracture mechanics, the relation between strain energy release rate and the change of kinetic energy was analyzed. A static simulation method for dynamic crack growth was proposed, which transformed the change of kinetic energy into the change of input fracture toughness (G_IC^*). G_IC^* was calculated according to the evolution of fracture resistance in the tests and input into ABAQUS for the simulation of crack growth using virtual crack closure technique (VCCT). A three-dimensional finite element model was constructed to simulate the dynamic delamination growth from crack onset to arrest, and the complex mechanical behavior in the unstable crack growth was studied.