The grey-white phase interfaces of tin have been studied using high-resolution electron microscopy (HREM). The lattice fringe images revealed the interfacial regions of the transitional atomic arrangement from grey tin (α-phase) into white tin β-phase). The orientation relationship obtained from the lattice fringe image between α- and β-phases is that the (011) plane of grey tin is parallel to the (001) plane of white tin and the [211] direction of grey tin is nearly parallel to the [010] direction of white tin. A model of the α → β transformation mechanism is proposed on the basis of the atomic arrangement observed by HREM. The model indicates that the α → β transformation of tin is accomplished partly by martensitic transformation and partly by atomic motion that is massive in nature.