Tough pitch copper has poor strength at high temperatures and contains much oxygen, which causes difficulty to obtain good joints with diffusion bonding. The authors have reported that roughness on the bonding surfaces and their contact timing were important for diffusion bonding on oxygen free copper. In the present study, solid state diffusion bonding on tough pitch copper was carried out under low pressures with investigating these factors. The specimens were significantly deformed with raising the bonding pressure, while voids at the bonded interface were scarcely changed. Tensile strength of the specimens was increased with decreasing the surface roughness. However, all the specimens fractured at the bonded interfaces and the tensile strengths were reduced in comparison with those of oxygen free copper. Relatively large dimples were partly observed on the fractured surfaces, which would suggest that oxide inclusions existed at the bonded interface. The tensile strength was further improved when contact between bonding surfaces was delayed until the temperature was raised up to bonding condition. The specimens prepared by polishing with emery paper #4000 and buffing fractured at the base metal. The large dimples were not completely observed on the fractured surfaces in such the specimens. The tensile strength was also increased with chemical polishing prior to bonding. Grain boundary migration across the bonded interface remarkably occurred with decreasing the surface roughness and was observed only in the specimens fabricated with the delayed contact procedure. On the other hand, the fracture could occur at the base metal even in the specimens that the grain boundary migration hardly originated.