For understanding deformation behavior in terms of microstructural aspects, ferrite-bainite steels were analyzed by the micro-grid method and FEA using unit cell model. Using the micro-grid method, it was directly observed that significant localization of strain occurred in ferrite phase adjacent to the ferrite/bainite boundary. From the FE results, higher internal stress was calculated in larger volume fraction of hard phase. Under further deformation, strain hardenability decrease due to lower stress distribution caused by plastic deformation in bainite. From the observation and analysis of the microscopic deformation behavior, a plastic instability condition of ferrite-bainite dual-phase steels was estimated.