Abstract: The rafting procedure of γ' phase in single crystal DD402 for DD402/FGH95 diffusion couples bonded by hot isostatic pressing(HIP) was studied.A new rafting mechanism of γ' phase was suggested.The driving force of rafting γ' phase was analyzed and a physical model of the rafting procedure of γ' phase was constructed.The relations of the width of rafted γ' phase pieces with the original size,volume fraction and constitutional element diffusion flux of γ' phase were discussed in details.The results show that the rafted reason of γ' phase in the single crystal is the continuous directional growth of γ' phase driven by diffusion of its constitutional elements.The rafted process includes two stages,one is the directional connect of γ' particles,and the other is the plainness process of γ' pieces.The width and direction of rafted γ' pieces were closely related to the original size and crystal orientation of γ' phase.