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We report our investigation of the block boundary and sub-block boundary strengths of lath martensite evaluated through a micro-bending test. The sub-block boundaries contribute very little to the macroscopic strength of the lath martensite. In contrast, the presence of a block boundary in the specimen greatly increased the strength. In addition, the block boundary induced a serrated flow and load drop after yielding in the load–displacement curve. The load drop and serrated flow were attributable to dislocation pile-up and subsequent propagation of dislocations across the block boundary. In a microstructural observation of specimens after deformation, we found that a block boundary significantly restricts the motion of dislocations, while a sub-block boundary does not. We concluded that the block boundary is the most effective grain boundary for strength in lath martensite.