An investigation was made of the relation between microstructure and fracture characteristics of a low carbon tempered martensite. It was found in a previous paper that the cleavage crack traverses many martensite plates or laths in a linear fashion. On the analogy of the ferritic steel, the effective grain is defined as a region in which cleavage crack runs nearly straight. It is shown that the microstructure corresponding to the effective ferrite grain is the ‘co-variant packet’ extending throughout a bundle of the martensite. The reason why the structure consisting of fine martensites fractures with a large cleavage facet was found to be due to the fact that most of martensite plates or laths in a packet have the same crystallographic orientation and the cleavage plane {100}_α is quasi-continuous throughout the packet.