Abstract In this study, the Numerical Manifold method (NMM) is extended to investigate the effects of micro-defects on the dynamic mechanical properties of rock under different strain rates. The displacement decomposition technique is incorporated in the NMM to estimate the mixed mode stress intensity factors at the crack tip. A dynamic crack growth criterion is also incorporated in the NMM for crack growth analysis. The developed NMM is first validated by a simple sliding crack model. The developed model is then applied to investigate the effects of the micro-cracking properties such as initial micro-crack length, initial micro-crack inclination angle and initial micro-crack separation distance on the dynamic mechanical properties of a granite under different strain rates ranging from 10−4/s to 100/s. The effect of confining stress on the granite dynamic strength is also investigated. Simulation results illustrated that the initial micro-crack length and the confining stress have a significant effect on the dynamic strength. The effect of micro-crack separation distance, on the other hand, is heavily dependent on the ratio of separation distance to the initial micro-crack length.