Microstructure, strain gradient and strain rate are key factors that influence fracture of heterogeneous brittle materials. The present work for the first time unifies effects of these factors in an analytical dynamic fracture criterion based on fundamental micromechanics. By using an energy-based two-scale theory accounting for micro-inertia, we rigorously derive an formulation for microscopic dynamic energy release rate involving additive contributions of macroscopic strain, strain gradient and strain rate. The coefficients of the formulation are correlated to microstructural length scale and are calculated based on integrals of the first order microscopic cell solutions. The two-scale formulation of energy release rate, along with the Griffith law for a single micro-crack, results in a novel dynamic fracture model. The remarkable feature of this modeling approach is that, without extra phenomenological ad hoc hypotheses, all microstructural length scale, strain gradient and strain rate effects are natural consequences of the unified two-scale theory.Capabilities of the dynamic fracture model for predicting coupled effects of microstructure size, strain gradient and strain rate on brittle fracture behaviors are confirmed by numerical simulations. The model is also well validated against experimental results. Especially, finite element simulations based on the model well reproduce free surface velocity profiles, fracture zones and spall strengths measured in series of dynamic spalling fracture experiments.