It is well known that duplex stainless steels having refined grain, structure exhibit superplasticity. In this paper, the diffusion of boron in the superplastic duplex stainless steels has been investigated. Boron diffuses through the interior of the grains in superplastic duplex stainless steels. On the other hand, in case of normal duplex stainless steel having the same chemical concentration, boron mainly diffuses along the grain boundaries and does not diffuse through the interior of the grains. Investigations using Auger electron microscopy for super plastic duplex stainless steels revealed, the Cr concentration is much higher than Fe concentration in boron diffused areas. These results indicate that superplasticity activates diffusion into the interiors of grains. Further, it seems that boron preferentially reacts with Cr rather than Fe. Moreover the distribution of Fe and Cr is changed by the diffusion of boron.