The influence of ultra fine grained (UFG) and coarse grained (CG) microstructure of the titanium alloy Ti-6Al-4V on the strength of a diffusion bonded (DB) joint was studied using a laboratory DB fixture and a new shear test rig. The DB process was carried out at 725 °C and 825 °C during 2 and 4 h in a vacuum furnace. Coarsening of grain structure resulting from different DB cycles was quantified. The chain pores were observed at 725 °C for both microstructure conditions bonded during 2 h. The increase of bonding time up to 4 h leads to subsequent elimination of the pores. The UFG samples bonded at 725 °C showed a higher level of the shear strength than CG samples for both bonding times. The CG material demonstrated the highest shear strength after 4 h of DB bonding at 825 °C. The increase of the creep deformation of UFG samples when compared to the CG condition was observed as a result of DB at of 725 °C during 4 h.