18R martensite phase was observed in the bcc matrix in a Fe-25.8wt%Mn-7.4wt%Al-0.11wt%C alloy, after cooling from high temperature. The crystal structure of 18R martensite is determined from the SAD and high resolution imaging techniques. The 18R martensite has an orthorhombic lattice with the lattice constant a = 0.448 nm, b = 0.259 nm, and c = 3.865 nm and can be described as a 18R(42̄)3 rather than a 18R(51̄)3 structure. The martensitic crystallography such as orientation relationship, habit plane, shape strain direction and magnitude of lattice invariant shear can be calculated using CRAB phenomenological martensitic theory. The calculated crystallography fits very well with the experimental crystallography. The Burgers vector of anticoherency dislocation and shape strain direction were confirmed by using two beam technique and computer image simulation. Martensitic crystallography analyses suggest that the bcc/18R(42̄)3 interface may be more energetically favorable than the bcc/18R(51̄)3 interface.