Transmission electron microscopy (TEM) in situ straining experiments were carried out on duplex ferritic-austenitic steel. Localization of strain as a common feature during slip transfer from the austenite to the ferrite has been noticed. The character of strain localization depends on the orientation relationship between the phases. A possible influence of the TEM specimen geometry on the mechanism of plastic deformation of the ferritic-austenitic steel was considered by comparing the results with those of in situ straining experiments have been performed in a 1 MV high-voltage TEM instrument.