The crystal structure of martensite in carbon steels is proposed to consider as a defined system of the lattices blocks. Each block consists of four lattices with carbon atom at the central axis of the block. There are lattices without carbon atoms in steels. Carbon concentration in steel determines the quantity of the blocks with C atom. Edges of the lattices in the blocks and outside them have different sizes that depend on the distance to the block axis with carbon atom. It is necessary to consider the martensitic crystal structure in the carbon steels as pseudotetragonal or pseudocubic. Reasons of the large width of the XRD lines of the martensitic phase in the carbon steels are considered and their explanation is proposed. The number of the $c$ edges, which are changed in the same way, determines the diffraction lines width of the martensite in the carbon steels.