Self-interstitial diffusion in -iron is investigated using an embedded-atom-method potential and molecular-dynamics simulations. Curved Arrhenius plot is obtained for the temperature dependence of diffusion coefficients, which is well explained by the superposition of two transition processes among the two allotropic states of self-interstitial defects, the reformation of h110i dumbbell into another h110i configuration and one-dimensional solitonic propagation of a crowdion on h111i axis.