Microstructure characterization of twin structures were performed at 300 kV using a Titan G60-300 TEM equipped with double spherical aberration (Cs) correctors for both the probe-forming and image-forming lenses. The convergent semiangle was chosen as 20.8 mrad, and a large inner collection angle was set as 50 mrad. The composition of the stainless steel in the present study is shown in Table 1. We choose 316F stainless steel because it provides a large number of MnS inclusions for analysis. This steel sample was made by Nippon Steel and Sumikin Stainless Steel Corporation [15]. The steel sample was hot-rolled (>1000 °C) into rods with diameter of 1 cm.