Several　alloys　containing　weak　or　strong　interfaces　were　cyclically　loaded　for　different　cycles.　In　situ　observation　on　specimen　surfaces　was　made　to　collect　information　on　fatigue.　Weak　interface　plays　role　of　weak　link　micro　region.　From　where,　the　elastic　interfacial　debonding　always　originates　in　the　early　stage　of　fatigue,　leading　to　fatigue　damages　or　straight　forward　turning　into　fatigue　fracture.　Strong　interfaces　act　as　obstacles　to　cyclic　slip　and　at　which　dislocations　pile　up,　of　which　the　former　hinders　and　so　delays　fatigue　progress,　while　the　later　increases　in　stress　concentration.　Interface　designs　resistant　to　fatigue　through　weak　interface　strengthening,　strong　interface　strengthening　and　toughening,　and　the　interface　spheroidization　have　been　made,　and　which　has　been　confirmed　to　be　effective　in　practice.