The structure of FCC: BCC boundary, which is expected to be partially coherent and to be defined by Nishiyama-Wasserman orientation relationship,is resimulated by computer. The results show that the misfit dislocation is not the necessary structural element on the partially coherent FCC:BCC boundary,and introducing the structural ledges in two directions brings about an increase in the interfacial coherency and a decrease in the interfacial energy. A structural model of FCC:BCC partially coherent interface is made,which replenishes with and amends the R-A model.