A series of load-relaxation tests were, conducted on Ti-6Al-4V alloy with a transformed microstructure to investigate high temperature deformation mechanisms. The. flow stress-vs.-strain rate curves were well fit with the inelastic deformation equation describing grain matrix deformation (dislocation glide + dislocation climb). However, for the heavily deformed (epsilon approximate to 1.2) specimens, the operation of grain-boundary sliding as well as the grain matrix deformation was evidenced. The grain boundary sliding rate was found to be most rapid for the microstructure with the thinnest alpha laths/platelets.