The strain-hardening behavior of two recently developed double forged and K-doped tungsten grades in the 300–2000 °C range was analyzed applying a phenomenological model describing the evolution of the flow stress as a function of the dislocation density. The applied model allowed establishing a correlation between the strain hardening curvature and the size of microstructural features controlling the dislocation multiplication. The obtained results demonstrated that plastic deformation was controlled by the resistance of the low angle grain boundaries below 1000 °C and the high angle grain boundaries at 1500 °C and above. The experimental results obtained at different loading rates showed that thermal activation was essential for the passage of dislocations through grain boundary interfaces at 1000 °C and above. The limitations of the applied model and need for further development of the physical model accounting for stress- and temperature-induced grain growth are discussed.