Real-time in situ neutron diffraction was performed on an austenitic stainless steel under cyclic loading at room temperature. The evolutions of individual phase stresses during martensitic transformation were derived from the lattice parameters and volume fraction based on Rietveld refinements and Hooke’s law, while the phase-specific dislocation densities were elucidated by the single peak broadenings. Both results reveal that the increasing content of martensite phase, instead of individual phase strengthening, should be accounted for the remarkable secondary cyclic hardening.