GaN　nanomaterials,　as　one　of　the　most　important　third-generation　semiconductor　materials,　have　attracted　wide　attention.　In　this　study,　GaN　nanowires　with　square　cross　section　were　successfully　prepared　by　microwave　plasma　chemical　vapor　deposition　system.　The　diameters　of　nanowires　are　from　300　to　500　nm　and　the　lengths　from　15　to　20　mu　m.　The　results　show　that　the　cross　section　of　nanowires　could　be　transformed　from　triangle　into　square　by　adjusting　the　ratio　of　Mg　to　Ga　in　source　materials.　X-ray　diffraction(XRD)result　indicate　that　the　structure　of　GaN　nanowires　are　agree　with　the　hexagonal　wurtzite.　X-ray　photoelectron　spectroscopy　(XPS)　rusult　show　that　a　certain　amount　of　Mg　and　O　impurities　incoporated　in　the　square-shaped　GaN　nanowires.　Transmission　electron　microscopy　(TEM)　result　suggested　that　square-shaped　GaN　nanowires　had　high　crystallinity　with　a　growth　direction　of　[0　(1)　over　bar　10].　The　ratio　of　source　materials-　and　time-depented　growth　mechanism　was　also　studied.　It　was　suggested　that　the　transformation　of　the　cross　section　from　triangle　to　square　structure　should　be　derived　from　the　growth　mechanism　change　from　vapor-liquid-solid(VLS)process　to　vapor-solid(VS)process.　The　doped　Mg　increased　the　growth　rate　of　the　nanowires　sidewalls,　which　led　to　a　symmetrically　growth　of　GaN　nanowires　along　the　twin　boundaries.　GaN　nanowires　gradually　transformed　to　square　structure　by　auto-catalytic　growth.　Moreover,　the　property　of　field　emission　were　further　investigated.　The　results　showed　that　the　turn-on　electric　field　of　square-shaped　GaN　nanowires　was　5.2　V/m　and　a　stable　field　emission　property　at　high　electric　field.　This　research　provides　a　new　method　for　the　preparation　of　squareshaped　GaN　nanowires　and　a　prospective　way　for　the　design　and　fabrication　of　novel　nano-scale　devices.