Debye rings obtained by synchrotron X-ray diffraction were analyzed for investigating structural changes caused by stress-induced martensitic transformation and reverse transformation of a polycrystalline austenitic Fe-Mn-Si shape memory alloy. The chemical composition of the shape memory alloy was Fe-28 mass%Mn-6 mass%Si-5 mass%Cr. The results showed that a part of the austenitic γ phase was transformed to a martensitic ε phase by room-temperature tensile deformation, and the ε phase was reversely transformed by subsequent heating. Diffraction intensities in Debye rings changed non-uniformly on tensile deformation and heating, indicating that occurrences of the stress-induced and reverse transformation depend on the crystallographic orientations of grains with respect to the tensile direction. The optimum recovery strain induced by the reverse transformation was obtained for a sample deformed by about 10% tensile strain, which was consistent with the structural changes caused by the reverse transformation. X-ray diffraction lines were shown to be broadened by tensile strain. This indicated that irreversible deformation due to dislocations restricted the reverse transformation, leading to the optimum recovery strain.