It is known that lath martensite, despite its high yield strength, shows a very weak ductility. It was clarified that lath martensite exhibits significant anisotropy in local deformation behavior and that lath martensite is a composite of two kinds of grains with soft orientation (SO) and hard orientation (HO), depending on the active slip system, in-lath and out-of-lath slip system. To understand the microscopic mechanism of plastic deformation in lath martensite, the in-situ ECCI technique was conducted to investigate the microscopic dislocation movement in HO and SO. As results, at the early stage of the plastic deformation, dislocation density seems to drastically increase in HO, while slightly decreased in SO by the formation of cell structure. The edge component of the dislocations with the largest Schmid factor started moving first. In HO, this movement would form a high-density parallel screw dislocation array, while in SO, it induced cell structure formation.