© 2019 Acta Materialia Inc. In-situ neutron diffraction measurements during tensile tests at low temperatures of a low alloy steel containing retained austenite (γ) have been performed. Evolutions of phase fractions and phase stresses were analyzed and discussed with the progress of deformation. The role of γ in the steel during deformation at low temperatures was observed not to directly in the contribution to the strengths but in the improvement of the elongation by transformation of γ to martensite -and in the increasing of the work-hardening rate by an increase in the phase fraction of martensite and the work hardening of martensite.