Tailoring　mechanical　properties　of　the　nanowire　(NW)　with　intricate　composite　structure　helps　to　design　nanodevices　with　novel　functionalities.　Here,　we　performed　in　situ　tensile　deformation　electron　microscopy　for　the　evaluation　of　the　mechanical　properties　of　the　focused　ion　beam　(FIB)　irradiated　silicon　nitride　(Si3N4)　nanowires　(NWs).　Young＇s　modulus　of　the　FIB-fabricated　NWs　was　mediated　between　the　range　of　522　and　65　GPa　by　modifying　the　shell　thickness　of　the　core-shell　structure.　The　ion-beam-induced　amorphization　is　found　to　induce　the　structural　transition　from　an　utter　crystalline　state　to　a　composite　NW　with　an　amorphous　shell,　which　results　in　a　brittleto-ductile　transition　and　an　unexpected　plastic　deformation.　These　results　have　practical　implications　for　optimizing　nanostructures　with　the　desired　mechanical　properties,　which　are　of　fundamental　relevance　in　designing　and　fabricating　nanomechanical　devices.