Abstract. The in-situ straining of tetragonal martensite of Ni-Mn-Ga alloy was studied in a transmission electron microscope (TEM) JEM 1200EX equipped with a double tilt straining stage. The sample was analyzed before and after the straining experiments using the conventional as well as high resolution TEM (Tecnai F20 G2 200kV FEG). Before straining, the martensitic structure of the samples consisted of thermally induced self-accommodated multivariants. During the in-situ straining, detwinning processes were recorded. The volume fraction of the twin variants preferably oriented to the applied tensile force increased at the expense of the less favorably oriented ones. The movement of the twin boundaries was followed in detail. The analysis performed after straining suggested that the detwinning processes occurred by movement of twinning dislocations. The interface between two twinned bands acted as a nucleation source for emitting the twinning dislocations.