To increase the acceptance of directmetal laser sintered Ti6Al4V(Extra Low Interstitial—ELI) in industry, analytical models that can quantitatively describe the interrelationships between the microstructural features, field variables, such as temperature and strain rate, and the mechanical properties are necessary. In the present study, a physical model that articulates the critical microstructural features of grain sizes and dislocation densities for use in predicting the mechanical properties of additively manufactured Ti6Al4V(ELI) was developed. The flow stress curves of different microstructures of the alloy were used to obtain and refine the parameters of the physical model. The average grain size of a microstructure was shown to influence the athermal part of yield stress, while the initial dislocation density in a microstructure was seen to affect the shape of the flow stress curve. The viscous drag effect was also shown to play a critical role in explaining the upturn of flow stress at high strain rates. The microstructure-based constitutive model developed and validated in this article using experimental data showed good capacity to predict the high strain rate flow properties of additively manufactured Ti6Al4V(ELI) alloy.