The behavior of retained austenite during cyclic heat treatment and deformation in medium carbon Mn-Si steel was investigated using electron microscope. The orientation relation between the austenite and martensite satisfies both K-S and N-W relationships. Austenite was retained as thin films when quenched from γrange and as isolated islands when quenched from (α＋γ) range. The amount of retained austenite tends to decrease with increased quench temperature upto 1100℃. The retained austenite was mostly strain induced transformed to martensite in the early stage (1-5%) of tensile deformation The strain hardening exponent and total elongation were increased with increased volume fraction of retained austenite.