Microstructural changes in a structural martensitic steel upon low-cyclic fatigue (LCF) deformation have been investigated. The micromechanics of plastic deformation and accompanying effects have been studied at the scale of martensite laths and packets with the aid of transmission electron microscopy (TEM). It has been shown that with increasing LCF deformation, changes in both the morphology and the internal structure of martensite occur. The changes are manifested in the form of a refinement of the structural units of martensite. It has been revealed that in the limits of a packet the fatigue deformation occurs inhomogeneously. However, the laths of the same orientation are deformed equally and almost simultaneously. The influence of the dimensions of former austenite grains and orientation of packets on the LCF process has been considered. The mechanics of the fatigue plastic deformation on the nano-, meso-, and microlevels and the processes that accompany this deformation have been studied.