In order to understand more completely the formation of strain-induced martensite, phase structures were investigated both before and after plastic deformation, using austenitic stainless steels of various chemical compositions (carbon C=0.007-0.04 mass% and molybdenum Mo=0-2.10 mass%) and varying pre-strain levels (0-30%). Although the stainless steels consisted mainly of gamma austenite, two martensite structures were generated following plastic deformation, comprising epsilon and alpha' martensite. The martensitic structures were obtained in the twin deformation and slip bands. The severity of martensite formation (epsilon and alpha') increased with increasing C content. It was found that alpha' martensite was formed mainly in austenitic stainless steel lacking Mo, whereas a high Mo content led to a strong epsilon martensite structure, i.e. a weak alpha' martensite. The formation of alpha' martensite occurred from gamma austenite via e martensite, and was related to the slip deformation. Molybdenum in austenitic stainless steel had high slip resistance (or weak stress-induced martensite transformation), because of the stacking fault energy of the stainless steel affecting the austenite stability. This resulted in the creation of weak alpha' martensite. Models of the martensitic transformations gamma (fcc)-> epsilon (hcp)-> alpha' (bcc) were proposed on both the microscopic and nanoscopic scales. The alpha' martensite content of austenitic stainless steel led to high tensile strength; conversely, epsilon martensite had a weak effect on the mechanical strength. The influence of martensitic formation on the mechanical properties was evaluated quantitatively by statistical analysis.