Effects of alloying elements and thermomechanical treatment on mechanical and magnetic properties of Cr-Ni austenitic stainless steel have been investigated. N and Si increase hardness for annealed and cold rolled conditions. Aging treatment, following cold rolling, further increases hardness. Specially, Si exerts a substantial increase in hardness by aging treatment. On the other hand, Ni, Mn, Cr and N increase γ stability, while Si lowers γ stability against formation of strain induced martensite (α'). Nonmagnetism can be maintained by controlling γ stability, depending upon a degree of cold rolling. Based on the results, high strength nonmagnetic stainless steel can be obtained for Si and N containing Cr-Ni stainless steel with sufficiently high γ stability. Effects of Si on strain age hardening and α' transformation behaviors are discussed.