Austenitic stainless steels (A) despite having unique properties are less used in various industries due to their low yield strength. Grain refinement is the most effective way to increase strength and toughness at the same time. Martensite thermomechanical process is a promising method to produce nano/ultrafine grained metastable A. This process involves cold rolling to form strain-induced martensite followed by reversion annealing to achieve nano/ultrafine grained austenite. According to the conditions imposed during cold rolling (strain, strain rate, rolling temperature and austenite grain size), the martensite formation rate and therefore the obtained martensite morphology can be changed. In this work, effect of the martensite morphology in AISI 201 ASS with 50 and 90% thickness reduction on microstructure formation during annealing at different temperatures and times were investigated. The result showed that after 50% cold rolling, the dominant morphology was lath martensite, while it was cell-dislocation structure without any martensite laths after 90% reduction. By decreasing saturation strain and increasing strain, the volume fraction of cell-dislocation structure increased and the crystal size of this structure decreased from 50 nm for 50% to 10 nm for 90% reduction. It was found that during annealing at all temperatures, the austenite nucleation rate in the cell-dislocation structures was higher than that of the lath martensite due to higher number of nucleation sites. For annealing at 900 ?C, the lath martensite was fully reversed after 10 min, but this time was about 2 min for the cell-dislocation structure. The reversion mechanism at all temperatures was determined diffusional mechanism (nucleation and growth). Due to the larger austenite grains formed from lath martensite, lower driving force and also prevention of the residual martensite from rapid grain growth of austenite, growth of austenite grains formed from the lath martensite was lower than the cell-dislocation martensite. By annealing at 700 and 800 ?C, the austenite grain size formed from the cell-dislocation structure was finer. During the early stages of annealing at 900 ?C, the reversed austenite grain size formed from the cell-dislocation martensite was smaller, however, it was larger than the austenite formed from lath martensite in later stages. Under this annealing condition, the smallest austenite grain size with suitable austenite volume fraction was 120 nm (with 1050 MPa yield strength and 30% elongation) for the 90% cold rolled specimen after annealing for 30 s, while it was 500 nm (with 830 MPa yield strength and 35% elongation) for the 50% cold rolled steel after annealing for 60 s. Key words: 201 Austenitic stainless steel, Martensite thermomechanical process, Martensite morphology, lath martensite, cell-dislocation structure, reversion annealing.