The superplastic behaviors of a high specific strength steel (HSSS) with dual-phase microstructure and ultrafine grains have been investigated under a temperature range of 873-973 K and at a wide strain rate range of 10(-4)-10(-1)/s. The ultrafine grained HSSS exhibits excellent superplastic properties. The microstructure observations at interrupted strains for tests under temperature of 973 K and at strain rate of 10(-3)/s have provided evidences of different mechanisms for two stages. At the first stage (strain range from 0% to 400%), the superplastic flow is attributed to the diffusional transformation from fcc austenite phase to intermetallic compound B2 phase coupled with grain boundary sliding. While intragranular dislocation activities should be the dominant mechanism for the second stage (strain range from 400% to 629%) due to the increased realistic strain rate by diffusive necking. The grain sizes of both phases are observed to be relatively stable and remain always sub micron level during the high temperature tensile deformation, facilitating the superplastic flow.