Taking advantage of in-situ fracture testing method inside a transmission electron microscope, crack evolution in a low alloy steel under low triaxiality conditions is studied and the interaction between cementite particles and the crack is revealed. It is found that the ferrite matrix is the major void initiation site due to the low stress triaxiality in the TEM sample, which contrasts the behavior under plane strain conditions in bulk specimens, where voids are typically found to initiate by decohesion at the particle/matrix interface. This work reveals that fracture behavior proceeds differently under low triaxiality conditions, such as in thin films, and demonstrates the possibility to avoid interface decohesion and thereby to enhance ductility in steels.