TWIP (TWinning Induced Plasticity) steel is one of the advanced steels with attractive mechanical properties.The typical composition of TWIP steel includes a large amount of manganese with some aluminum and silicon.Previous study has shown that TWIP steel exhibits high strength with adequate elongation at high strain rates,so that TWIP steel is desired to be applied for automotive use.However,there are few studies concerning the deformation behaviors aimed to make clear the TWIP effect in TWIP steel.In this study,static tensile deformation behaviors of an Fe-30Mn-3Al-3Si TWIP steel and a SUS310S one were studied by in situ neutron diffraction during tensile deformation.In terms of mechanical properties obtained by the static tensile tests,the TWIP steel showed better balance of tensile strength and uniform elongation than the 310S steel.The angular dispersion neutron diffraction with a wavelength of 0.16 nm was performed during stepwise tensile testing by using a neutron diffractometer for residual stress analysis (RESA) at the Japan Atomic Energy Agency.A specimen was extended in a step by step manner and neutron diffraction profiles of (111),(200) and (311) for austenite were obtained at each step.The diffraction peak,lattice plane spacing,lattice plane strain and so on were determined by the profile analysis as a function of applied stress.The changes of lattice plane strain for austenite in the TWIP and 310S steels indicated several deformation stages in the tensile deformation and can be discussed the difference of intergranular stress between the two samples. TWIP (TWinning Induced Plasticity) steel is one of the advanced steels with attractive mechanical properties. The typical composition of TWIP steel includes a large amount of manganese with some aluminum and silicon. Previous study has shown that TWIP steel exhibits high strength with adequate elongation at high strain rates, so that TWIP steel is desired to be applied for automotive use. 30Mn-3Al-3Si TWIP steel and a SUS310S one were studied by in situ neutron diffraction during tensile deformation. The terms of mechanical properties obtained by the static tensile tests, the TWIP steel showed better balance of tensile strength and uniform elongation than the 310S steel The angular dispersion neutron diffraction with a wavelength of 0.16 nm was performed during stepwise tensile testing by using a neutron diffractometer for residual stress analysis (RESA) at the Japan Atomic Energy Agency. A specimen was extended in a step by step manner and neutron diffraction profiles of (111), (200) and (311) for austenite were obtained at each step. diffraction peak, lattice plane strain, so on the determined by the profile analysis as a function of applied stress. the changes of lattice plane strain for austenite in the TWIP and 310S steels indicated several deformation stages in the tensile deformation and can be discussed the difference of intergranular stress between the two samples.