Microstructural changes during the martensitic transformation from face centred cubic (FCC) to body centred cubic (BCC) in an Fe–31Ni alloy were observed by scanning electron microscopy (SEM) with a newly developed Peltier stage available at temperatures to −75 °C. Electron channelling contrast imaging (ECCI) was utilized for the in situ observation during cooling. Electron backscatter diffraction (EBSD) analysis at ambient temperature (20 °C) after the transformation was performed for the crystallographic characterization. A uniform dislocation slip in the FCC matrix associated with the transformation was detected at −57 °C. Gradual growth of a BCC martensite was recognized upon cooling from −57 °C to −63 °C.