In order to clarify the mechanism of fatigue fracture, fatigue crack propagation tests using Cu single crystals (which have face- centered cubic lattice ; f. c. c.), as a first step, were conducted in the previous paper. In the present paper, the same tests using 3% Si-steels (which have body-centered cubic lattice ; b. c. c., and some grain boundaries), as a second step, are conducted. With SEM observations of fracture surfaces, it is shown that fatigue crack in f. c. c. and b. c. c. propagates microscopically on the active slip plane which is characteristic of each crystal system, but not on the mechanical principal stress plane. This mechanism can not be explained by the Laird model, which is thought to be appropriate in continuum mechanics, and needs an expansion of the Neumann model. Relation between fatigue crack propagation rate and stress intensity factor range is investigated. In f. c. c. and b. c. c., an active slip system interferes intricately with the others, and fatigue crack propagation rates depend much on this mechanism. Effects of grain boundaries in fatigue crack propagation are also investigated. When plastic zone at crack front reaches a grain boundary, some changes in fracture surfaces and crack propagation rates are observed.