Well-defined uniaxial micro-tensile tests were performed on lath martensite with different types of substructure boundaries (block, sub-block and lath boundaries) tilted with respect to the loading direction. A characteristic deformation mechanism of lath martensite is hereby identified, i.e. sliding along the substructure boundaries. This boundary sliding can occur at all types of boundaries at relatively low stresses. Internal boundaries not only strengthen lath martensite, as well established in literature, but under favorable orientations also mitigate plastic deformation. The overall plastic deformation results from the competition of crystallographic slip with boundary sliding.