Understanding　the　compositional　distribution　of　ternary　nanowires　is　essential　to　build　the　connection　between　nanowire　structures　and　their　potential　applications.　In　this　study,　we　grew　epitaxial　ternary　InGaAs　nanowires　with　high　In　concentration　on　GaAs　{111}(B)　substrates.　Our　detailed　electron　microscopy　characterizations　suggest　that　the　grown　ternary　InGaAs　nanowires　have　an　extraordinary　core　shell　structure　with　In-rich　cores　and　Ga-enriched　shells,　in　which　both　nanowire　cores　and　shells　showed　compositional　gradient.　It　was　found　that　In-rich　nanowire　cores　are　formed　due　to　the　Ga-limited　growth　environment,　caused　by　the　competition　with　the　spontaneous　InGaAs　planar　layer　growth　on　the　substrate　that　consumes　more　Ga　than　the　nominal　Ga　concentration　during　the　growth.　Moreover,　the　composition　gradient　in　the　nanowires　cores　and　shells　is　a　result　of　strain　relaxation　between　them.　Our　optoelectronic　property　measurements　from　prototype　nanowire　devices　show　a　remarkable　photoresponsivity　under　the　near-infrared　illumination.　This　study　provides　a　new　approach　for　designing　and　realizing　complex　nanowire　heterostructures　for　high-efficiency　nanowire-based　systems　and　devices.