To investigate the stability of retained austenite under plastic deformation and the change of volume fractions of each phase, the microstructural characterization such as shape and orientation of retained austenite as uniaxial tension tests, including HR-EBSD observations, for TRIP steel made by POSCO have been carried out. In-situ and ex-situ mechanical test with EBSD measurement revealed the deformation sequence of multi phase steels and predicted the stability of retained austenite with its size. The granular type (>1mum) of retained austenite transformed faster than that of lath or film type (<1mum), which is well known from TEM measurement.