Lath martensite is the key constituent in advance steels that provides the overall strength. Martensite is known as hard and brittle, but recent evidence shows significant plasticity before fracture. The exactly reason is undisclosed but should relate to the underlying lath microstructure. Therefore, we studied the influence of sub-block and block boundaries on martensite plasticity through uni-axial tensile testing of individual micro-constituents, i.e. single block specimens and specimens with a single through-thickness block boundary parallel, perpendicular and at 45° to loading. A unique micro-tensile methodology was developed included micro-specimen fabrication with minimal FIB damage, EBSD at top and bottom surfaces, a home-built highly-sensitive uni-axial tensile tester, and in situ microscopic slip trace analysis. Interestingly, all specimens showed extensive plasticity before fracture and no cleavage, however, strong differences are observed. Detailed analysis of the rich experimental data shows that not only the block but also the sub-block boundaries show boundary strengthening following a Hall-Petch relation, in the case that the easiest slip systems are crossed by these boundaries. However, for boundaries oriented under 45°, often easy glide is observed along the boundary reducing the strength, possibly caused by retained austenite films at the boundaries. TEM analysis of the boundary structure is ongoing.