In order to clarify the mechanism of fatigue fracture, fatigue crack propagation tests using Cu single crystals, as a first step, are conducted. Six types of specimens, crack-crystal orientation relation is different in each, are used. And SEM observations of fracture surfaces are conducted. It is shown that fatigue crack in single crystal propagates microscopically on the active slip plane which is characteristic of each crystal system, 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 expansion of the Neumann model. Relation between fatigue crack propagation rate and stress intensity factor range is also investigated. In single crystal, an active slip system interferes intricately with other ones, and fatigue crack propagation rates depend much on this mechanism. Therefore, Δ K , a macro-mechanical parameter, is not useful for explaining the phenomenon in such single crystals.