Cold-rolled and annealed ferrite-martensite dual phase (DP) steel sheets are useful for automotive applications because of their excellent balance of strength and elongation. Although martensite in ferrite-martensite DP steels has important roll in the mechanical properties, the crystallography and microstructure of lath martensite in DP steels have not been investigated in detail. In this paper, the crystallography and microstructure of lath martensite in a ferrite-martensite DP steel were studied using electron microscopy and electron diffraction analysis, and the crystallographic orientation relationship between the ferrite and martensite was analyzed. The main results are as follows: (1) The lath martensite in DP steels consists of some packets, each of which contains several blocks. This structure is the same as that of lath martensite in fully martensitic steels. (2) The common parallel close-packed plane of the martensite laths in a packet tend to be parallel to the close-packed plane of the adjacent ferrite grain, whose fraction is about 30%.