Orientation relationships (ORs) play a crucial role in the phase transformation of face-centered cubic austenite (gamma-fcc) to body-centered cubic martensite (alpha '-bcc) upon plastic deformation. So far, ORs between gamma-fcc and alpha '-bcc are reported frequently; however, the correlation to the transformation sequences is not clear. Here, the gradient structured austenitic stainless 304 steel prepared by ultrasonic nanocrystalline surface modification was examined to clarify the ORs that were involved in the strain-induced martensitic transformation (SIMT) behaviors. The results showed changes in the ORs from Nishiyama-Wassermann (N-W) to Kurdjumov-Sachs (K-S) and Pitsch with an increase in depth from the top-treated surface. The OR of alpha '-bcc formed directly on gamma-fcc was identified as N-W, while those via the hexagonal closed packed epsilon-plate (epsilon-hcp) were the K-S and Pitsch as they were nucleated at the single and two epsilon-plates intersections, respectively. Thus, the SIMT sequence was the direct gamma -> alpha ' with the presence of the N-W OR in the high strain depth, while it was the gamma -> epsilon -> alpha ' as the existence of K-S and Pitsch ORs in the low strain depth. The findings provide essential data for the in-depth and comprehensive study of the polymorphic martensitic transformation mechanisms in various steels and alloys.