Abstract The mechanical behavior of hafnium is influenced by texture, strain rate, temperature, chemistry, and stress state; however the effects of microstructure remain poorly understood. As is observed in other group IVb elements, such as titanium and zirconium, deformation in hafnium at quasi-static and dynamic strain rates occurs through a combination of prismatic slip, pyramidal slip, and twinning. For titanium and zirconium, the mechanical behavior has been previously shown to be highly dependent upon the average grain size of the material. Here, the effects of grain size on the mechanical response of high purity hafnium tested in compression are systematically probed and the flow stress and work hardening behavior is correlated with the substructural and textural evolution during deformation. The influence of strain rate, temperature, and texture as a function of the microstructure is examined and Hall–Petch constants for hafnium are calculated.