X-ray and TEM investigations were performed to clarify plastic deformation in cold streching of micro-duplex stainless steel. The X-ray line broading was obtained by the Garrod and Auld method which utilizes a single reflection only. The main results obtained were summarized as foolows; Most of the deformation occurred in the soft austenite phase and the two phases tended to deform more or less at the equal stress. In the case of aging at 748K, the hardness of ferrite phase increased, owing to the precipitate of Cr-rich phase and the two phases tended to deform with the equal strain. The reason was due to the decrease in continuity of softer austenite phase. By aging, twinning occurred in coarse grained steel. But it was restricted remarkably when the grains of both phases were fine. The main reason of this was due to the fine dispersion of the soft austenite phase. When the domain size by the X-ray method and the magnitude of inter-slip region (cell size) by the transmission electron microscopy method were compared, the former was larger than the latter by a factor of 2.