There　has　been　a　long-standing　controversy　on　how　dislocations　interact　with　interfaces.　Here　we　report　in-situ　observations　that　in　a　Cu-brass　heterostructured　TEM　film　Frank-Read　sources　are　the　primary　dislocation　sources.　They　were　dynamically　formed　and　deactivated　throughout　the　deformation　in　grain　interior,　which　has　never　been　reported　before.　This　observation　indicates　that　strain　gradient　near　interfaces　cannot　be　quantitatively　related　to　the　density　gradient　of　geometrically　necessary　dislocations,　and　it　was　primarily　produced　by　Frank-Read　source　gradient　instead　of　dislocation　pile-ups.　These　findings　provide　new　insights　on　how　to　design　heterostructured　interfaces　to　enhance　mechanical　properties.　[GRAPHICS]