Gallium　nitride　(GaN)　nanowires　(NWs)　were　grown　on　Si(100)　substrates　at　different　hydrogen　gas　flow　rates,　using　plasma-enhanced　chemical　vapor　deposition.　The　size　and　morphology　of　the　GaN　NWs　could　be　modulated　by　controlling　the　hydrogen　atmosphere.　The　diameters　of　the　GaN　NWs　ranged　from　53　to　221　nm,　and　their　morphology　transformed　from　a　hexagonal　prism　to　a　triangular　pyramid　upon　changing　the　hydrogen　atmosphere　conditions.　The　modulation　effects　originated　from　the　competitive　equilibrium　between　surface　and　interfacial　diffusion　induced　by　hydrogen　in　the　vapor-liquid-solid　growth　mechanism.　The　photoluminescence　spectra　of　the　different　GaN　NWs　may　have　originated　from　two　metabolic　peaks,　which　were　closely　related　to　their　nanostructures.　These　findings　provide　a　simple　and　convenient　method　to　modulate　the　structure　of　GaN　NWs　and　may　further　the　application　of　GaN　in　nano-optoelectronic　devices.