Tensile behavior of a nano-TRIP steel with 0.4 μm grain size showing tensile strength of 1326 MPa and total elongation of 23% was studied under in situ neutron diffraction. In spite of ultrafine grained structure, this steel realized a high work-hardening leading to a sufficient amount of uniform elongation. In the beginning of tensile deformation, Lüders band appeared similarly to many ultrafine grained materials. The stress induced martensitic transformation was found to occur during the Lüders deformation. A double-peak fitting was applied to the overlapped neutron diffraction profile for martensite and ferrite and then the stress partitioning behavior among ferrite, retained austenite and martensite were tracked during tensile deformation, revealing that the work-hardening after Lüders deformation was caused by higher load sharing of deformation induced martensite.