Lath martensitic steels are widely used iron-based alloys with outstanding mechanical properties. However, despite its high yield strength, it shows 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 macro evaluation of dislocation before and after tensile deformation for tempered SCM415 steel will be estimated by CMWP fitting, and an in-situ ECCI technique will be conducted to investigate the microscopic dislocation movement in HO and SO. As results, at the early stage of the plastic deformation in tempered low-carbon steel lath martensite, the evolution of the dislocation structure during tensile deformation observed by ECCI corresponds well with CMWP fitting results. From CMWP fitting, the overall dislocation density increased to about 1.5 times of that before deformation, and the proportion of screw dislocations was indicated to increase after the deformation. From ECCI observation, dislocation density drastically increased 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.