We　demonstrate　a　structural　modulation　method　of　GaN　nanowires　(NWs)　by　microwave　plasma　chemical　vapor　deposition.　This　method　is　based　on　the　self-assembled　growth　mode　without　a　harmful　gas　source　and　could　easily　achieve　structures　with　a　remarkable　range　of　geometries　and　sizes　by　regulating　the　plasma-phase　conditions.　The　results　show　that　we　are　able　to　modulate　GaN　NWs　with　a　large　scale　of　sizes　of　lengths　from　similar　to　2.2　to　52.4　mu　m　and　diameters　from　similar　to　22　to　1000　nm.　Due　to　introduce　the　high-density　plasma　phase,　we　propose　a　new　growth　and　modulation　model　of　GaN　NWs　via　combining　the　competition　and　equilibrium　of　the　interface　effect,　the　diffusion　effect,　and　the　surface　effect.　We　found　that　the　critical　diameter　defined　by　the　surface　energy　has　a　noteworthy　correlation　with　the　growth　of　NWs　due　to　the　significant　effect　of　nitrogen　plasma　on　the　surface　of　NWs.　In　addition,　the　photoluminescence　performance　including　the　emission　peak　position,　intensity,　and　full　width　at　half-maximum　can　be　well　regulated　by　the　structural　effects　of　GaN　NWs.　This　modulation　provides　an　economical,　flexible,　and　environmentally　friendly　route　to　single-crystalline　GaN　NWs　and　their　devices.　Furthermore,　the　combined　growth　effect　model　could　provide　new　physical　and　chemical　insight　into　the　growth　mechanism　of　GaN　NWs　controlled　by　high-density　plasma.