Metastable stainless steels are promising engineering materials demonstrating good corrosion resistance and good formability but they have also relative low yield strength. Among the different strengthening mechanisms, grain refinement is the only method to improve both strength and toughness simultaneously. Producing nano/submicron grain size metastable austenitic stainless steel by an advanced thermomechanical process is the aim of this thesis.Metastable austenitic stainless steels undergo a strain-induced martensitic transformation, where the metastable austenite phase is transformed to the thermodynamically more stable ??-martensite phase due to the plastic deformation. The strain induced martensitic transformation enhances the work hardening of metastable austenitic stainless steels. This thesis concentrated on the effects of the strain-induced martensitic transformation on the mechanical properties of AISI 304L Metastable austenitic stainless steel, focusing on increasing yield strength by grain refinement.The experiments were carried out on steel (AISI 304L). Mechanical testing was performed by means of uniaxial tensile tests. The ??-martensite volume fractions were measured with a Ferritescope.X-ray diffraction was used for the phase identification of the test materials. Microstructure investigations were carried out by means of the scanning electron microscopy and optical metallography. Industrial hot rolled and annealed samples whose thickness were 10 mm, were cold rolled using different thickness reductions (from 10% to 90%). Cold reductions at 0 °C of 10 - 90% were carried out with inter-pass cooling. In order to obtain homogeneous austenitic microstructures, 90% deformed samples were annealed at different temperatures (700 °C– 900 °C) and times (30 s–1800 s). The grain size before cold reduction was approximately 36 ?m.Nano-crystalline grains of about 300 nm were obtained in the AISI304L metastable austenitic steel by a thermomechanical process consisting of cold rolling at 0 °C and 90% reduction and annealing at 700 °C and 300 min.The nano-structured austenitic steel exhibits not only high strength ( 1010 MPa ) but also good elongation (above 50%).