The high temperature deformation mechanisms of two phase alpha+beta alloy and a near-alpha alloy were investigated, and compared within the framework of inelastic-deformation theory. For this purpose, load-relaxation tests were conducted on the two alloys at temperatures of 750 similar to 900 degrees C. The flow stress-vs.-strain rate curves for both alloys were well fit with inelastic deformation equations describing dislocation glide and grain boundary sliding. The amount of grain boundary sliding resistance was higher in the near-cc alloy rather than the two phase alpha+beta alloy due to difficulty in stress relaxation at triple junction region.