In the present study, the micro-sized cantilever-beam type specimens containing only one block of lath martensite were fabricated, and change in deformation microstructure inside a block with strain was observed directly by scanning electron microscopy and transmission electron microscopy. A number of slip bands appeared in the fixed end of the specimen by deformation. The propagations of slip bands, however, terminate due to the gradient of strain inside the specimen. The direction of slip bands changed during the propagation by the low angle boundaries inside the block. The shear localized region becomes narrow with an increase in strain. Furthermore, the width of laths increases greatly at the large strain region. The increase in width of laths is attributed to the disappearance of some initial lath boundaries by deformation.